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Duality of roaming mechanism in H 2 CO The phenomenon of roaming in chemical reactions (that is, bypassing the minimum energy pathway from unlikely geometries) has attracted a great deal of attention in the chemical reaction dynamics community over the past decade and still demonstrates unexpected results. Using velocity-map imaging of state-selected H 2 products of H 2 CO photodissociation, Quinn et al. discovered the bimodal structure of rotational distribution of the other product fragment, CO. Quasiclassical trajectories showed that this bimodality originates from two distinctive reaction pathways that proceed by the trans or cis configuration of O–C–H⋯H, leading to high or low rotational excitations of CO, respectively. Whether such a mechanism is present in the many other chemical reactions for which roaming reaction pathways have been reported is yet to be determined. Science , this issue p. 1592

Norrish Type I (NTI) α-bond cleavage is the dominant photolysis mechanism in small carbonyls and is an important source of radicals in the troposphere. In nonsymmetric species two cleavages are possible, NTIa and NTIb, forming larger and smaller alkyl radicals, respectively. For a data set of 20 small, atmospherically relevant carbonyls we predict NTIa and NTIb thresholds on the S0, S1, and T1 electronic states. The calculated NTIa T1 thresholds give a mean absolute deviation (MAD) of 5.8 kJ/mol with respect to the available experimental thresholds of five carbonyls. In addition, the intrinsic barrier heights to dissociation on the S0, S1, and T1 electronic states are predicted. We find RI-B2GP-PLYP/def2-TZVP calculations on S0 and unrestricted RI-B2GP-PLYP/def2-TZVP calculations on T1 give MADs of 6.1 kJ/mol for S0 asymptotic energies and 6.3 kJ/mol for S0 → T1 0-0 excitation energies, with respect to available experimental data. A composite method is used to determine S1 thresholds, with bt-STEOM-CCSD/cc-pVQZ calculation of vertical excitation energies and TD-RI-B3LYP/def2-TZVP calculations on S1, which achieves a MAD of 7.2 kJ/mol, with respect to experimental 0-0 excitation energies. Our calculations suggest, with the exception of bifunctional carbonyls and enones, NTI reactions on S1 are unlikely to be important at tropospherically relevant photolysis energies (

Carbonyls are among the most abundant volatile organic compounds in the atmosphere. They are central to atmospheric photochemistry as absorption of near-UV radiation by the C=O chromophore can lead to photolysis. If photolysis does not occur on electronic excited states, non-radiative relaxation to the ground state will form carbonyls with extremely high internal energy. These “hot” molecules can access a range of ground state reactions. Up to nine potential ground state reactions are investigated at the B2GP-PLYP-D3/def2-TZVP level of theory for a dataset of 20 representative carbonyls. Almost all are energetically accessible under tropospheric conditions. Comparison with experiment suggests the most significant ground state dissociation pathways will be concerted triple fragmentation in saturated aldehydes, Norrish type III dissociation to form another carbonyl, and H2-loss involving the formyl H atom in aldehydes. Tautomerisation, leading to more reactive unsaturated species, is also predicted to be energetically accessible and is likely to be important when there is no low-energy ground state dissociation pathway, for example in α,β-unsaturated carbonyls and some ketones. The concerted triple fragmentation and H2-loss pathways have immediate atmospheric implication to global H2 production and tautomerisaton has implication to the atmospheric production of organic acids.

The first experimental observation of the primary photochemical channel of acetaldehyde leading to the formation of ketene (CH2CO) and hydrogen (H2) molecular products is reported. Acetaldehyde (CH3CHO) was photolysed in a molecular beam at 305.6 nm and the resulting H2 product characterized using velocity-map ion (VMI) imaging. Resonance-enhanced multiphoton ionization (REMPI), via two-photon excitation to the double-well EF 1Σ+g state, was used to state-selectively ionize the H2 and determine angular momentum distributions for H2 (ν = 0) and H2 (ν = 1). Velocity-map ion images were obtained for H2 (ν = 0 and 1, J = 5), allowing the total translational energy release of the photodissociation process to be determined. Following photolysis of CH3CHO in a gas cell, the CH2CO co-fragment was identified, using Fourier transform infrared spectroscopy, by its characteristic infrared absorption at 2150 cm−1. The measured quantum yield of the CH2CO + H2 product channel at 305.0 nm is ϕ = 0.0075 ± 0.0025 for both 15 Torr of neat CH3CHO and a mixture with 745 Torr of N2. Although small, this result has implications for the atmospheric photochemistry of carbonyls and this reaction represents a new tropospheric source of H2. Quasi-classical trajectory (QCT) simulations on a zero-point energy corrected reaction-path potential are also performed. The experimental REMPI and VMI image distributions are not consistent with the QCT simulations, indicating a non reaction-path mechanism should be considered.

Carbonyls are among the most abundant volatile organic compounds in the atmosphere, and their C=O chromophores allow them to photolyse. However, carbonyl photolysis reactions are not restricted to the excited state: the C=O chromophore allows relaxation to, and reaction on, the ground state, following photon absorption. In this paper, the energetic thresholds for eight ground state reactions across twenty representative carbonyl species are calculated using double-hybrid density functional theory. Most reactions are found to be energetically accessible within the maximum photon energy available in the troposphere, but are absent in contemporary atmospheric chemistry models. Structure–activity relationships are then elucidated so that the significance of each reaction pathway for particular carbonyl species can be predicted based upon their class. The calculations here demonstrate that ground state photolysis pathways are ubiquitous in carbonyls and should not be ignored in the analysis of carbonyl photochemistry.

Carbonyls are among the most abundant volatile organic compounds in the atmosphere, and their C=O chromophores allow them to photolyse. However, carbonyl photolysis reactions are not restricted to the excited state: the C=O chromophore allows relaxation to, and reaction on, the ground state, following photon absorption. In this paper, the energetic thresholds for eight ground state reactions across twenty representative carbonyl species are calculated using double-hybrid density functional theory. Most reactions are found to be energetically accessible within the maximum photon energy available in the troposphere, but are absent in contemporary atmospheric chemistry models. Structure–activity relationships are then elucidated so that the significance of each reaction pathway for particular carbonyl species can be predicted based upon their class. The calculations here demonstrate that ground state photolysis pathways are ubiquitous in carbonyls and should not be ignored in the analysis of carbonyl photochemistry.

Photolysis thresholds are calculated for the Norrish Type II (NTII) intramolecular γ-hydrogen abstraction reaction in 22 structurally informative carbonyl species. The B2GP-PLYP excited state S1 and T1 thresholds agree well with triplet quenching experiments. However, many linear-response methods deliver poor S1 energetics, which is explained by a S1/S0 conical intersection in close proximity to the S1 transition state. Multiconfigurational CASSCF calculations confirm a conical intersection features across all carbonyl classes. Structure–activity relationships are determined that could be used in atmospheric carbonyl photochemsitry modelling. This is exemplified for butanal, whose NTII quantum yields are too low when used as a ‘surrogate’ for larger carbonyls, since butanal lacks the γ-substitution that stabilises the 1,4- biradical. Reaction on T1 dominates only in species where the S1 thresholds are high — typically ketones. The α, β-unsaturated carbonyls cannot cleave the α–β bond, causing them to photoisomerise. A concerted S0 NTII mechanism is calculated to be viable and may explain the recent detection of NTII photoproducts in the photolysis of pentan-2-one below the T1 threshold.

Photolysis thresholds are calculated for the Norrish Type II (NTII) intramolecular γ-hydrogen abstraction reaction in 22 structurally informative carbonyl species. The B2GP-PLYP excited state S1 and T1 thresholds agree well with triplet quenching experiments. However, many linear-response methods deliver poor S1 energetics, which is explained by a S1/S0 conical intersection in close proximity to the S1 transition state. Multiconfigurational CASSCF calculations confirm a conical intersection features across all carbonyl classes. Structure–activity relationships are determined that could be used in atmospheric carbonyl photochemsitry modelling. This is exemplified for butanal, whose NTII quantum yields are too low when used as a ‘surrogate’ for larger carbonyls, since butanal lacks the γ-substitution that stabilises the 1,4- biradical. Reaction on T1 dominates only in species where the S1 thresholds are high — typically ketones. The α, β-unsaturated carbonyls cannot cleave the α–β bond, causing them to photoisomerise. A concerted S0 NTII mechanism is calculated to be viable and may explain the recent detection of NTII photoproducts in the photolysis of pentan-2-one below the T1 threshold.

We approach the problem of predicting excitation energies of diverse, larger (5–6 carbons) carbonyl species central to earth’s tropospheric chemistry. Triples contributions are needed for the vertical excitation energy (Evert), while EOM-CCSD//TD-DFT calculations provide acceptable estimates for the S1 relaxation energy (Erelax), and (TD-)DFT suffices for the S0 → S1 zero-point vibration energy correction (∆EZPVE). Perturbative triples corrections deliver Evert values close in accuracy to full iterative triples EOM-CC calculations. The error between EOM-CCSD and triples-corrected E vert values appears to be systematic and can be accounted for with scaling factors. However, saturated and α,β-unsaturated carbonyls must be treated separately. Double-hybrid S0 minima can be used to calculate Evert with negligible loss in accuracy, relegating the O(N5) expense of CCSD to only single-point energy and excitation calculations. This affordable protocol can be applied to all volatile carbonyl species. E0−0 predictions do overestimate measured values by ∼8 kJ/mol due to a lack of triples contribution in E relax, but this overestimation is systematic and the mean unsigned error is within 4 kJ/mol once this is accounted for.

We approach the problem of predicting excitation energies of diverse, larger (5–6 carbons) carbonyl species central to earth’s tropospheric chemistry. Triples contributions are needed for the vertical excitation energy (Evert), while EOM-CCSD//TD-DFT calculations provide acceptable estimates for the S1 relaxation energy (Erelax), and (TD-)DFT suffices for the S0 → S1 zero-point vibration energy correction (∆EZPVE). Perturbative triples corrections deliver Evert values close in accuracy to full iterative triples EOM-CC calculations. The error between EOM-CCSD and triples-corrected E vert values appears to be systematic and can be accounted for with scaling factors. However, saturated and α,β-unsaturated carbonyls must be treated separately. Double-hybrid S0 minima can be used to calculate Evert with negligible loss in accuracy, relegating the O(N5) expense of CCSD to only single-point energy and excitation calculations. This affordable protocol can be applied to all volatile carbonyl species. E0−0 predictions do overestimate measured values by ∼8 kJ/mol due to a lack of triples contribution in E relax, but this overestimation is systematic and the mean unsigned error is within 4 kJ/mol once this is accounted for.

Calculation of the gas phase proton affinity of CH4, PA(CH4), involves characterisation of the CH5+ ion whose fluxional nature requires reevaluation of some assumptions routinely made for more normal molecules. Here we determine the PA(CH4; 0 K) using quantum diffusion Monte Carlo (QDMC) simulations on a previously developed CCSD(T)/aug-cc-pVTZ potential energy surface for CH5+. We obtain a value of 542.4 kJ/mol in reasonable agreement with the most recent measurements. The results obtained from standard thermochemical methods, and the limitations of these methods, are also discussed.

The first experimental observation of the primary photochemical channel of acetaldehyde leading to the formation of ketene (CH

A new approach for preventing zero-point energy (ZPE) violation in quasi-classical trajectory (QCT) simulations is presented and applied to H

Organic acids play a key role in the troposphere, contributing to atmospheric aqueous-phase chemistry, aerosol formation, and precipitation acidity. Atmospheric models currently account for less than half the observed, globally averaged formic acid loading. Here we report that acetaldehyde photo-tautomerizes to vinyl alcohol under atmospherically relevant pressures of nitrogen, in the actinic wavelength range, λ = 300–330 nm, with measured quantum yields of 2–25%. Recent theoretical kinetics studies show hydroxyl-initiated oxidation of vinyl alcohol produces formic acid. Adding these pathways to an atmospheric chemistry box model (Master Chemical Mechanism) demonstrates increased formic acid concentrations by a factor of ~1.7 in the polluted troposphere and a factor of ~3 under pristine conditions. Incorporating this mechanism into the GEOS-Chem 3D global chemical transport model reveals an estimated 7% contribution to worldwide formic acid production, with up to 60% of the total modeled formic acid production over oceans arising from photo-tautomerization., The concentration of formic acid in Earth’s atmosphere is under-predicted by atmospheric models. Here the authors show that acetaldehyde photo-tautomerizes to vinyl alcohol under tropospheric conditions, with subsequent oxidation via OH radicals supplying up to 60% of total modeled formic acid production over oceans.

Quantum thermodynamic parameters can be determined using path integral Monte Carlo (PIMC) simulations. These simulations, however, become computationally demanding as the quantum nature of the system increases, although their efficiency can be improved by using higher order approximations to the thermal density matrix, specifically the action. Here we compare the standard, primitive approximation to the action (PA) and three higher order approximations, the Takahashi-Imada action (TIA), the Suzuki-Chin action (SCA) and the Chin action (CA). The resulting PIMC methods are applied to two realistic potential energy surfaces, for H2O and HCN–HNC, both of which are spectroscopically accurate and contain three-body interactions. We further numerically optimise, for each potential, the SCA parameter and the two free parameters in the CA, obtaining more significant improvements in efficiency than seen previously in the literature. For both H2O and HCN–HNC, accounting for all required potential and force evaluatio...

The dynamics of CO production from photolysis of H

Fourier transform infrared spectra of isolated 1-propenol and 2-propenol in the gas-phase have been collected in the range of 900–3800 cm–1, and the absolute infrared absorption cross sections reported for the first time. Both cis and trans isomers of 1-propenol were observed with the trans isomer in greater abundance. Syn and anti conformers of both 1- and 2-propenol were also observed, with abundance consistent with thermal population. The FTIR spectrum of the smaller ethenol (vinyl alcohol) was used as a benchmark for our computational results. As a consequence, its spectrum has been partially reassigned resulting in the first report of the anti-ethenol conformer. Electronic structure calculations were used to support our experimental results and assign vibrational modes for the most abundant isomers, syn-trans-1-propenol and syn-2-propenol.

The correlated speed and rotational energy distributions of the CO fragment from photodissociation of CH3CHO have been measured at a range of wavelengths from 308 to 328 nm. The distributions are bimodal, showing low J, slow speed, and high J, fast speed components. The cold component disappears for λ > 325 nm. This threshold corresponds to C–H bond cleavage and we assign these CO products as arising from roaming of a H-atom about a CH3CO core. We attribute the hot component to CO formed through CH3-roaming. No evidence was observed for the presence of a transition state mechanism. This is the first time two distinct roaming channels have been observed from the same electronic state. The results support the growing understanding that roaming can be significant in chemical reactions and outweigh traditional pathways.

The concentrations of organic acids, key species in the formation of secondary organic aerosols, are underestimated by atmospheric chemistry models by a factor of ∼2. Vinyl alcohol (VA, CH2═CHOH, ethenol) has been suggested as a precursor to formic acid, but sufficient tropospheric sources of VA have not been identified. Here, we show that VA is formed upon irradiation of neat acetaldehyde (CH3CHO) in the actinic ultraviolet region, between 295 and 330 nm. Besides the well-known photochemical products CO and CH4, we infer up to a 15% quantum yield of VA at 20 Torr acetaldehyde pressure and a photolysis wavelength of 330 nm. The experiments confirm a recent model predicting phototautomerization of acetaldehyde to VA and imply that photolysis of small aldehydes and ketones could provide tropospheric sources of enols sufficient to impact organic acid budgets. We also report absolute infrared absorption cross sections of VA.

The spectrum for the photodetachment of electrons from H 3 O - to produce H 2 +OH or H+H 2 O is calculated with the rotating bond approximation (RBA). This method treats explicitly three active degrees of freedom (two stretching vibrations and one bending mode) in both H 3 O - and the neutral reactive product. Potential-energy surfaces based on new ab initio calculations in these same three degrees of freedom are used. Comparison with experimental photodetachment spectra demonstrates the accuracy of the model and confirms that the experiment accesses photodetachment from both the H - (H 2 O) isomer of H 3 O - , producing H+H 2 O, and the higher energy OH - (H 2 ) isomer yielding OH+H 2 . In the latter case, a progression of bands associated with different OH rotational states in the H 2 +OH product channel is predicted. The calculations suggest that the observed photodetachment spectrum is mainly providing information on the entrance and exit channels of the OH+H 2 →H 2 O+H potential-energy surface quite close to the transition state.

Finite temperature quantum and anharmonic effects are studied in H2-Li(+)-benzene, a model hydrogen storage material, using path integral Monte Carlo (PIMC) simulations on an interpolated potential energy surface refined over the eight intermolecular degrees of freedom based upon M05-2X/6-311+G(2df,p) density functional theory calculations. Rigid-body PIMC simulations are performed at temperatures ranging from 77 K to 150 K, producing both quantum and classical probability density histograms describing the adsorbed H2. Quantum effects broaden the histograms with respect to their classical analogues and increase the expectation values of the radial and angular polar coordinates describing the location of the center-of-mass of the H2 molecule. The rigid-body PIMC simulations also provide estimates of the change in internal energy, ΔUads, and enthalpy, ΔHads, for H2 adsorption onto Li(+)-benzene, as a function of temperature. These estimates indicate that quantum effects are important even at room temperature and classical results should be interpreted with caution. Our results also show that anharmonicity is more important in the calculation of U and H than coupling-coupling between the intermolecular degrees of freedom becomes less important as temperature increases whereas anharmonicity becomes more important. The most anharmonic motions in H2-Li(+)-benzene are the "helicopter" and "ferris wheel" H2 rotations. Treating these motions as one-dimensional free and hindered rotors, respectively, provides simple corrections to standard harmonic oscillator, rigid rotor thermochemical expressions for internal energy and enthalpy that encapsulate the majority of the anharmonicity. At 150 K, our best rigid-body PIMC estimates for ΔUads and ΔHads are -13.3 ± 0.1 and -14.5 ± 0.1 kJ mol(-1), respectively.

Unexpected and significant isotope exchange is observed in the near-threshold photodissociation of isopically labelled acetaldehyde. Theoretical modelling indicates that, at the lowest energies considered, an average of 20 H- or D-shifts occur before dissociation — evidence for extensive isomerization.

A new approach for preventing zero-point energy (ZPE) violation in quasi-classical trajectory (QCT) simulations is presented and applied to H2CO “roaming” reactions. Zero-point energy may be problematic in roaming reactions because they occur at or near bond dissociation thresholds and these channels may be incorrectly open or closed depending on if, or how, ZPE has been treated. Here we run QCT simulations on a “ZPE-corrected” potential energy surface defined as the sum of the molecular potential energy surface (PES) and the global harmonic ZPE surface. Five different harmonic ZPE estimates are examined with four, on average, giving values within 4 kJ/mol—chemical accuracy—for H2CO. The local harmonic ZPE, at arbitrary molecular configurations, is subsequently defined in terms of “projected” Cartesian coordinates and a global ZPE “surface” is constructed using Shepard interpolation. This, combined with a second-order modified Shepard interpolated PES, V, allows us to construct a proof-of-concept ZPE-corrected PES for H2CO, Veff, at no additional computational cost to the PES itself. Both V and Veff are used to model product state distributions from the H + HCO ! H2 + CO abstraction reaction, which are shown to reproduce the literature roaming product state distributions. Our ZPE-corrected PES allows all trajectories to be analysed, whereas, in previous simulations, a significant proportion was discarded because of ZPE violation. We find ZPE has little effect on product rotational distributions, validating previous QCT simulations. Running trajectories on V, however, shifts the product kinetic energy release to higher energy than on Veff and classical simulations of kinetic energy release should therefore be viewed with caution.

Isomeric oxo-bridged analogs of aza-trishomocubane sigma (sigma) receptor ligands were synthesized and shown to display a reduced affinity for the sigma receptor. In the case of phenethyl derivative 4, there was a concomitant introduction of high-affinity for the alpha(2C) adrenergic receptor, and moderate affinity for the dopamine transporter. Molecular modeling was undertaken to rationalize these results.

Density functional theory (DFT) and ab initio methods are used to investigate the interaction of one, two and three hydrogen molecules with Li + -doped benzene, a model for lithium-doped carbon-based and metal organic framework materials. M05-2X is found to be the best DFT method considered, reproducing MP2 and CCSD(T) H 2 binding energies to Li + -doped benzene. The M05-2X results also agree with H 2 binding energies previously obtained in an extended model of Li atom-doped MOF-5. These calculations suggest H 2 binding in Li-doped materials is, primarily, a local interaction, implying that model compounds can be used to describe these systems.

The effects of the enantiomers of a number of flexible and cis -constrained GABA analogues were tested on GABA C receptors expressed in Xenopus laevis oocytes using two-electrode voltage-clamp electrophysiology. (1 S ,2 R )- cis -2-Aminomethylcyclopropane-1-carboxylic acid ((+)-CAMP), a potent and full agonist at the ρ1 (EC 50 ≈ 40 μM, I max ≈ 100%) and ρ 2 (EC 50 ≈ 17 μM, I max ≈ 100%) receptor subtypes, was found to be a potent partial agonist at ρ3 (EC 50 ≈ 28 μM, I max ≈ 70%). (1 R ,2 S )- cis -2-Aminomethylcyclopropane-1-carboxylic acid ((−)-CAMP), a weak antagonist at human ρ1 (IC 50 ≈ 890 μM) and ρ2 (IC 50 ≈ 400 μM) receptor subtypes, was also found to be a moderately potent antagonist at rat ρ3 (IC 50 ≈ 180 μM). Similarly, (1 R ,4 S )-4-aminocyclopent-2-ene-1-carboxylic acid ((+)-ACPECA) was a full agonist at ρ1 (EC 50 ≈ 135 μM, I max ≈ 100%) and ρ2 (EC 50 ≈ 60 μM, I max ≈ 100%), but only a partial agonist at ρ3 (EC 50 ≈ 112 μM, I max ≈ 37 %), while (1 S ,4 R )-4-aminocyclopent-2-ene-1-carboxylic acid ((−)-ACPECA) was a weak antagonist at all three receptor subtypes (IC 50 >> 300 μM). 4-Amino-( S )-2-methylbutanoic acid (( S )-2MeGABA) and 4-amino-( R )-2-methylbutanoic acid (( R )-2MeGABA) followed the same trend, with ( S )-2MeGABA acting as a full agonist at the ρ1 (EC 50 ≈ 65 μM, I max ≈ 100%), and ρ2 (EC 50 ≈ 20 μM, I max ≈ 100%) receptor subtypes, and a partial agonist at ρ3 (EC 50 ≈ 25 μM, I max ≈ 90%). ( R )-2MeGABA, however, was a moderately potent antagonist at all three receptor subtypes (IC 50 ≈ 16 μM at ρ1, 125 μM at ρ2 and 35 μM at ρ3). On the basis of these expanded biological activity data and the solution-phase molecular structures obtained at the MP2/6-31+G* level of ab initio theory, a rationale is proposed for the genesis of this stereoselectivity effect.

The quantitative structure–activity relationship (QSAR) model constructed in this work represents the first quantitative investigation into agonist binding at GABAC receptors. This model is based upon the three-dimensional structures of (g-aminobutyric acid (GABA) and 12 other biologically active GABA analogues. These structures are obtained by geometry optimization and conformational exploration at MP2/6-31CG* within the conductor-like screening solvation model (COSMO). The biological activity data are obtained from previous twoelectrode voltage clamp electrophysiological studies on recombinant GABAC r1 receptors expressed in Xenopus laevis oocytes. A QSAR model to predict GABAC agonist binding is constructed from molecular superposition data, generated by least-squares superposition of the stable aqueous phase conformations of GABA and its known biologically active analogues to the bioactive conformation of TACA, the most potent GABAC receptor agonist. A significant relationship is found between the root-mean-squared deviation in atomic position for the stable conformer of each GABA analogue most closely fitting the template TACA structure and the natural log of the normalized biological activity (R 2 Z0.91, p!0.0001), On the basis of molecular superposition and QSAR results, a pharmacophore model describing three-dimensional features and key interactions at the GABAC receptor binding site is proposed. As there exists no direct experimental knowledge of structure of the GABAC binding site, this approach provides a feasible and reliable alternative to gain insight into its three-dimensional structure. q 2005 Elsevier B.V. All rights reserved.

The influence of treatment of electron correlation, size of basis set and choice of solvation model on the predicted stabilization of zwitterionic phosphinic and phosphonic acid gamma-aminobutyric acid (GABA) analogues is investigated using ab initio molecular orbital methods and density functional theory. Density functional theory with the B3LYP functional and a composite basis set composed of the 6-31+G(2df) basis for phosphorus atoms and the 6-31+G(d,p) basis set for all other atoms is found to give an acceptable tradeoff between accuracy and computational expense. Either directly optimizing zwitterionic conformers within the conductor-like screening solvation model (COSMO) or pre-optimizing as dihydrates and then applying the COSMO solvation model give an acceptable treatment of solvation in terms of determining stable solvated structures, although directly optimizing within COSMO is simpler and less computationally expensive. With this protocol, cis-constrained phosphinic and phosphonic acid GABA analogues, which exhibit lower affinities for GABAC receptors, are found to possess only folded, intramolecularly hydrogen bonded low energy conformers. Trans-constrained analogues, on the other hand, exhibit higher affinities for GABAC receptors and are found to exist only as partially folded stable conformers. Conformationally flexible analogues can attain folded, partially folded or fully extended conformations and also have high biological activity. These results imply that the partially folded conformation is likely to be the most biologically active.

The solution-phase structures of a number of conformationally restricted gamma-aminobutyric acid (GABA) analogues are investigated at the MP2/6-31+G* level of theory, using both explicit water molecules and the conductor-like screening solvation model (COSMO) to model solvation. GABA analogues constrained in a cis conformation by either a double bond or cyclopropane ring have the potential to attain either folded, intramolecularly hydrogen-bonded, or partially folded conformations in solution. Systems constrained in a cis conformation by a cyclopentane or cyclopentene ring are more conformationally restricted and exist only in a folded, intramolecularly hydrogen-bonded form. GABA analogues constrained in a trans conformation by either a double bond or cyclopropane ring have the potential to adopt either partially folded or fully extended conformations in solution. Due to a lack of conformational flexibility, analogues that are constrained in a trans conformation by a cyclopentane or cyclopentene ring attain only partially folded conformations. Like GABA, conformationally flexible GABA analogues possess a large number of stable rotamers, and may exist in any or all of these conformations in aqueous solution. The structures of these analogues provide an essential foundation for subsequent structure-activity analysis of ligand binding at GABA receptors and transporters. This work is therefore expected to facilitate the design and development of new biologically active GABA analogues to treat GABA-related neurological disorders.

The stabilization of γ-aminobutyric acid (GABA) zwitterions in aqueous solution is investigated at the HF/6-31+G*, B3LYP/6-31+G*, and MP2/6-31+G* levels of theory using explicitly bound water molecules to model short-range solvent effects and the conductor-like screening solvation model (COSMO) to estimate long-range solvent interactions. B3LYP and MP2 yield similar structures, relative energies and overall trends whereas HF theory does not provide a realistic description of GABA. The only approaches yielding zwitterionic structures consistent with experiment are the application of COSMO to GABA·2H2O or GABA·5H2O. An accurate description of aqueous phase GABA therefore requires both explicit interaction with at least two water molecules and long-range dielectric interactions with the solvent. Both types of interaction preferentially stabilize zwitterionic over neutral structures and stabilize extended zwitterions with respect to folded conformers. No stable neutral GABA·5H2O tautomers are obtained, sugges...

The accuracy of expanding the response of a molecule to an external electric field, E , as a power series in the field is investigated in the model hydrogen-bonded complex, ClH:NH 3 . Even at field strengths large enough to cause dramatic structural change in the complex, both the structure and vibrational frequencies are quantitatively predicted using only terms linear in E . These results suggest that knowledge of the zero-field molecular potential energy and dipole moment surfaces may be sufficient to accurately model the interactions of molecules in a wide range of external electric fields.

EOM-CCSD calculations are performed to evaluate two-bond 35Cl−15N spin−spin coupling constants (2hJCl-N) for ClH:NH3 complexes. Coupling constants for structures in external electric fields of 0.0000, 0.0055, and 0.0150 au are investigated as models for complexes with traditional, proton-shared, and ion-pair hydrogen bonds. Two-dimensional coupling constant surfaces are constructed at these field strengths in the NH and ClH distances, and expectation values, 〈2hJCl-N〉, are calculated for ground and selected excited vibrational states of the dimer- and proton-stretching modes from the corresponding anharmonic wave functions. Single-point values, 2hJCl-N, are also calculated at the equilibrium geometry for each field strength and at the geometry corresponding to the ground-state expectation values of the NH and ClH bond lengths. Coupling constants evaluated in the presence of the electric field are referred to as explicit 〈2hJCl-N〉 and 2hJCl-N. Implicit 〈2hJCl-N〉 and 2hJCl-N are evaluated from the zero-fiel...

The dynamics of CO production from photolysis of H2CO have been explored over a 8000 cm−1 energy range (345 nm–266 nm). Two-dimensional ion imaging, which simultaneously measures the speed and angular momentum distribution of a photofragment, was used to characterise the distribution of rotational and translational energy and to quantify the branching fraction of roaming, transition state (TS), and triple fragmentation (3F) pathways. The rotational distribution for the TS channel broadens significantly with increasing energy, while the distribution is relatively constant for the roaming channel. The branching fraction from roaming is also relatively constant at 20% of the observed CO. Above the 3F threshold, roaming decreases in favour of triple fragmentation. Combining the present data with our previous study on the H-atom branching fractions and published quantum yields for radical and molecular channels, absolute quantum yields were determined for all five dissociation channels for the entire S1←S0 abs...

Ab initio calculations are carried out to investigate the effects of external electric fields and chemical substitution on the properties of complexes stabilized by N−H−N hydrogen bonds. Two-dimens...

Vibrational effects on NMR shielding constants and nuclear spin–spin coupling constants have been investigated in a model hydrogen-bonded complex, CNH:NCH. Expectation values of the spin–spin coupling constant 2 h J N – N , obtained from a two-dimensional EOM–CCSD/(qzp,qz2p) surface, and of the isotropic proton shielding constant σH, obtained from a two-dimensional MP2/(qzp,qz2p) surface, are presented as functions of vibrational state. The expectation values have been computed from anharmonic dimer- and proton-stretching vibrational wavefunctions obtained from a two-dimensional MP2/aug′-cc-pVTZ potential surface. Equilibrium values, ground-state expectation values, and thermally averaged values at 298 K of 2 h J N – N and σH are compared.

This article provides a summary of our studies of hydrogen-bonded complexes during the decade of the 90's. These studies began with systematic investigations of the methodological dependence of the computed structures and binding energies of these complexes. The MP2/6-31+G(d,p) level of theory was identified as the minimum level required to obtain reliable structures, while reliable energetics required larger polarized split-valence basis sets that include diffuse functions. While the experimental frequency shift of the A–H stretching band upon formation of an A–H–B hydrogen bond could also be reproduced at MP2/6-31+G(d,p) for a variety of hydrogen-bonded complexes, significant discrepancies were observed for others, including complexes of HCl and HBr with ammonia, trimethylamine, and 4-substituted pyridines. Resolving these discrepancies became the primary focus of our work, and redefined our research efforts. We solved a model two-dimensional nuclear Schrodinger equation to obtain anharmonic dimer- and proton-stretching frequencies, modeled matrix effects with external electric fields, and characterized hydrogen bond types as traditional, proton-shared, and ion-pair. We were able to resolve the observed discrepancies between theory and experiment, and explain the rather disparate effects of matrices on the IR spectra of closely related complexes. We also initiated studies of the NMR properties of the chemical shift of the hydrogen-bonded proton, and the A–B spin–spin coupling constant across the A–H–B hydrogen bond. We demonstrated the dominance of the Fermi-contact term for determining coupling constants in complexes with N–H–N, N–H–O, O–H–O, and Cl–H–N hydrogen bonds, and the distance dependence of this term. We also showed that the IR anharmonic proton-stretching frequency and the NMR spin–spin coupling constant are spectroscopic fingerprints of hydrogen bond type, which provide information about intermolecular distances in hydrogen-bonded complexes.

Calculating F−F spin−spin coupling constants across hydrogen bonds has represented a significant challenge to theory. In this study, ab initio calculations have been carried out to evaluate vibrational effects on the F−F spin−spin coupling constant (2hJF-F) for FHF-. The coupling-constant surface 2hJF-F was generated at EOM-CCSD/(qzp,qz2p), and two-dimensional wave functions for the symmetric and asymmetric stretching vibrations were obtained from the CCSD(T)/aug‘-cc-pVTZ potential surface. The effect of the FHF- bending mode was examined using one-dimensional calculations along the normal coordinate for the bending motion. Although 2hJF-F is dominated by the Fermi-contact term in the region of the surface surrounding the equilibrium structure, the paramagnetic spin−orbit and spin dipole terms are important in determining the absolute value of 2hJF-F. In the ground vibrational state, the expectation value of the F−F distance increases, and the expectation value of 2hJF-F decreases to 212.7 Hz, significant...

MP2/aug‘-cc-pVDZ potential surfaces for the hydrogen-bonded complex ClH:pyridine have been generated without and with external electric fields. The zero-field, gas-phase structure of this complex is stabilized by a traditional Cl−H···N hydrogen bond. As the field strength increases, the equilibrium structure changes to that of a proton-shared hydrogen-bonded complex, which is close to quasi-symmetric at a field of 0.0040 au, and then an ion-pair complex at higher fields. Anharmonic dimer- and proton-stretching frequencies have been computed from each surface, and compared to experimental frequencies in Ar and N2 matrices. The computed results suggest that the hydrogen bond in ClH:pyridine is on the traditional side of quasi-symmetric in an Ar matrix, and on the ion-pair side in an N2 matrix. EOM-CCSD and MP2 calculations have been performed on the equilibrium structure at each field strength to obtain the 35Cl−15N spin−spin coupling constant across the hydrogen bond, and the chemical shift of the hydrogen...

Quantum and anharmonic effects are investigated in H2-Li(+)-benzene, a model for hydrogen adsorption in metal-organic frameworks and carbon-based materials. Three- and 8-dimensional quantum diffusion Monte Carlo (QDMC) and rigid-body diffusion Monte Carlo (RBDMC) simulations are performed on potential energy surfaces interpolated from electronic structure calculations at the M05-2X/6-31+G(d,p) and M05-2X/6-311+G(2df,p) levels of theory using a three-dimensional spline or a modified Shepard interpolation. These calculations investigate the intermolecular interactions in this system, with three- and 8-dimensional 0 K H2 binding enthalpy estimates, ΔH(bind) (0 K), being 16.5 kJ mol(-1) and 12.4 kJ mol(-1), respectively: 0.1 and 0.6 kJ mol(-1) higher than harmonic values. Zero-point energy effects are 35% of the value of ΔH(bind) (0 K) at M05-2X/6-311+G(2df,p) and cannot be neglected; uncorrected electronic binding energies overestimate ΔHbind (0 K) by at least 6 kJ mol(-1). Harmonic intermolecular binding enthalpies can be corrected by treating the H2 "helicopter" and "ferris wheel" rotations as free and hindered rotations, respectively. These simple corrections yield results within 2% of the 8-dimensional anharmonic calculations. Nuclear ground state probability density histograms obtained from the QDMC and RBDMC simulations indicate the H2 molecule is delocalized above the Li(+)-benzene system at 0 K.

Zhang acknowledges partial support from the Academic Research Grant No. RP3991603, The National University of Singapore.

Anharmonicity and matrix effects play important roles in determining the proton-stretching frequencies in hydrogen-bonded complexes of HCl and HBr with NH3 and N(CH3)3. These effects have been investigated through ab initio calculations carried out at MP2/aug‘-cc-pVDZ for complexes with HCl and at MP2/6-31+G(d,p) for complexes with HBr. The potential surfaces of these complexes are very anharmonic, since the region surrounding the global minimum may be very broad and relatively flat, or a second region of the surface, displaced from the global minimum, can be accessed in either the ground (v = 0) or the first excited (v = 1) state of the proton-stretching mode. As a result, two-dimensional anharmonic frequencies, particularly for the proton-stretching vibration, can be dramatically different from the corresponding harmonic frequencies. Moreover, the zero-point energy contribution to binding enthalpies based on harmonic vibrational frequencies can be significantly overestimated in some complexes. To model ...

The hydrogen bond has long been recognized as an important type of intermolecular interaction. Its infrared (IR) spectroscopic signature is the shift to lower frequency and the increase in intensity of the A-H stretching band upon formation of the A-H…B hydrogen bond. Ab initio calculations carried out with an appropriate wavefunction model and basis set, and using the harmonic approximation, can reasonably reproduce the shift of the A-H stretching band upon hydrogen bonding, if the equilibrium structure exists in a relatively deep potential well on the surface, so that both the V=0 and the V=1 vibrational states of the proton-stretching mode are confined within this well. However, if the equilibrium structure is found in a region of the surface which is broad and relatively flat, or if a second region of the surface can be accessed in either the V=0 or the V=1 vibrational state of the proton-stretching mode, then the harmonic approximation fails to describe the anharmonicity inherent in the surface. For ...

We present a method for expressing a potential energy surface (PES) for polyatomic molecules as an interpolation of local Taylor expansions in internal coordinates. This approach extends and replaces an earlier method which was only directly applicable to molecules of no more than four atoms. In general, the local Taylor expansions are derived from ab initio quantum calculations. Here, the methodology is evaluated by comparison with an analytic surface for the reactions H+CH4⇌H2+CH3. Approximately 1000–1300 data points are required for an accurate 12-dimensional surface which describes both forward and backward reactions, at the energy studied.

Ab initio calculations have been performed to investigate the structures and infrared spectra of the complexes FH:NH3, ClH:NH3, and BrH:NH3, and the effects of the presence of inert gas atoms on structures and spectra. Two-dimensional MP2/6-31+G(d,p) potential energy surfaces were constructed for the complexes XH:NH3, and model two-dimensional Schrodinger equations were solved for the proton stretching and dimer (heavy-atom) stretching modes. Although all complexes have equilibrium structures characterized by traditional hydrogen bonds, their infrared spectra differ significantly. In FH:NH3 both the ground (v=0) and first excited state for the proton stretching mode (v=1) are confined to the potential well describing the equilibrium structure. In this case the harmonic approximation is appropriate, and matrix effects are unimportant. In ClH:NH3 the v=1 proton stretching vibration accesses the more polar, proton-shared region of the potential surface. Here the harmonic treatment leads to a significant over...

We present a new method for expressing a molecular potential energy surface (PES) as an interpolation of local Taylor expansions. By using only Cartesian coordinates for the atomic positions, this method avoids redundancy problems associated with the use of internal coordinates. The correct translation, rotation, inversion, and permutation invariance are incorporated in the PES via the interpolation method itself. The method is most readily employed for bound molecules or clusters and is demonstrated by application to the vibrational motion of acetylene.

Ab initio calculations have been carried out to investigate the structure and infrared spectrum of BrH:NH3, and to resolve the discrepancies found between its computed harmonic and experimental spectrum. MP2/6-31+ G(d,p) and MP2/aug'-cc-pVDZ potential surfaces have been constructed, and a model two-dimensional nuclear vibrational problem has been solved to obtain the fundamental dimer stretching and proton stretching frequencies. These vibrational modes are strongly coupled anharmonic modes. The anharmonic proton stretching frequencies are lower than the harmonic by 1100 and 900 cm-1 respectively at the two levels of theory and are in better agreement with the experimental frequency. The argon matrix alters the potential energy surface by preferentially stabilizing the more polar proton-shared structure over the structure with a traditional hydrogen bond.

A simple model potential energy surface is constructed and used in both quasiclassical trajectory calculations and quantum vibrational close-coupling, infinite order sudden approximation calculations of collision-induced vibrational energy transfer from four vibrational states of S1 p-difluorobenzene. Classical and quantum state-to-state cross sections are compared for excitation of the two lowest energy vibrational states and collision with He or Ar. Qualitatively, the same trends are seen in both sets of results. Classical cross sections, however, are significantly larger at very low collision energies as a consequence of the binning procedures used to determine classical final states and, in the case of the Ar collider, as a result of the possible breakdown of the sudden approximation. Rotational excitation of the p-difluorobenzene molecule is also investigated and found to have only small effects on the dominant energy transfer channels. The theoretical results are compared with recent experimental results of Mudjijono and Lawrance [J. Chem. Phys. 104, 7444 (1996)]. The classical results, for the He, Ne, Ar, and Kr collision partners, show good agreement with experiment, reproducing the major energy transfer channels and the experimental collision partner dependence. Quantum results agree well with experiment for the He collider and are also used to assign experimentally ambiguous product states and to investigate vibrational energy transfer channels that are not experimentally observable. The propensity toward the transfer of multiple quanta of vibrational energy is analyzed and, in general, found to increase with the intermolecular well depth and with the mass of the collision partner. The He collision partner, however, behaves anomalously. © 1997 American Institute of Physics.

The photodissociation dynamics of H2CO molecules at energies bracketing the triple fragmentation threshold were investigated using velocity map ion imaging of the H-atom fragments. An algorithm was developed to model the experimental results as a two-step process: initially barrierless C-H bond fission on the S0 potential energy surface to form H + HCO, followed by secondary fragmentation of those HCO radicals with sufficient internal energy to overcome the small exit channel barrier on the HCO surface to form H + CO. Our model treats the first step using phase space theory (PST) and the second using a combined PST-impulsive model, with a tunneling correction. Experimentally, triple fragmentation reaches 25% of the radical (H + HCO) channel photochemical yield at energies about 1500 cm(-1) above the barrier for breaking the second bond. In addition, the triplet (T1) channel appears to reduce in importance after the barrier on the T1 surface is exceeded, slowly decreasing to10% of the total radical yield at higher energy. The double PST-impulsive model provides a good fit to the experimental H-atom speed and energy distributions for H2CO dissociation on S0 spanning7000 cm(-1) of available energy.

We describe a new, simple theory for predicting the branching fraction of products in roaming reactions, compared to the analogous barrierless bond dissociation products. The theory uses a phase space theory (PST) formalism to divide reactive states in the bond dissociation channel into states with enough translational energy to dissociate and states that may roam. Two parameters are required, ΔEroam, the energy difference between the bond dissociation threshold and the roaming threshold, and the roaming probability, Proam, the probability that states that may roam do roam rather than recombine to form reactants. The PST-roaming theory is tested against experimental and theoretical data on the dissociation dynamics of H2CO, NO3, and CH3CHO. The theory accurately models the relative roaming to bond dissociation branching fraction over the experimental or theoretical energy range available in the literature for each species. For H2CO, fixing ΔEroam = 146 cm(-1), the midpoint of the experimental bounds for the roaming threshold, we obtain Proam = 1. The best-fit value, ΔEroam = 161 cm(-1), is also consistent with the experimental bounds. Using this value, the relative roaming to dissociation branching ratios are predicted to be similar in D2CO and H2CO, consistent with experimental observation. For NO3, we fix ΔEroam = 258.6 cm(-1), the experimental threshold for NO + O2 production, and we model low-temperature experimental branching fractions using the experimental rotational and vibrational temperatures of Trot = 0 K and Tvib = 300 K. The best fit to the experimental data is obtained for Proam = 0.0075, with this very small Proam being consistent with the known geometric constraints to formation of NO + O2. Using Proam = 0.0075, our PST-roaming theory also accurately predicts the low-temperature NO yield spectrum and quantum yield data for room-temperature NO3 photolysis. For CH3CHO, we fix ΔEroam = 385 cm(-1), based on theoretical calculations, and obtain a best-fit value of Proam = 0.21, fitting to reduced dimensional trajectory calculations. These values of ΔEroam and Proam yield PST-roaming theory results that are also consistent with two experimental room-temperature data points. The combination of other kinetic theories and the PST-roaming theory will provide rate coefficients for roaming reactions.