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Organic excess electron compounds with significant nonlinear optical (NLO) properties are widely employed in optoelectronic applications. Herein, single-alkali metals with diaza[2.2.2] cryptand (M@crypt,M=Li, Na, and K) are investigated for optoelectronic and NLO properties by using the density functional theory. Thermodynamic and kinetic stabilities of present complexes are computed through interaction energy (Eint) and ab-initio molecular dynamic (AIMD) simulations. M@crypt complexes carry excess electrons and mimic molecular electrides. Quantum theory of atoms in molecules (QTAIM) analysis and reduced density gradient (RDG) spectra demonstrate the roles of the weak van der Waals (vdW) interactions between metal and complexant. The remarkable hyperpolarizability (βo) value up to 1.41 × 106 au may be credited to the presence of loosely bound excess electrons. The hyper Rayleigh scattering hyperpolarizability (βHRS) is recorded up to 1.31 × 106 au for the K@crypt. Furthermore, frequency-dependent first-order and second-order hyperpolarizability is more prominent at the applied frequency of ω = 0.042823 au. The electron localizing function (ELF) and localized orbital locator (LOL) analysis further disclose the nature of interaction between alkali metal and complexant. The TD-DFT method is adopted to get excited state parameters and absorbance properties. An electron density difference map (EDDM) is exploited to evaluate the orbital contributions in excited states. Hence, the studied electride may become a promising candidate for NLO materials. We anticipate that the present work will provide insight into further development of molecular electride for optoelectronic applications.

By means of highly accurate ab initio and dynamical calculations, we identify a suitable laser cooling candidate that contains a transition metal element, namely zinc monohydride (ZnH). The internally contracted multireference configuration interaction method is employed to compute the five lowest-lying Λ-S states of ZnH with the spin-orbit coupling effects included, and very good agreement is obtained between our calculated and experimental spectroscopic data. Our findings show that the position of crossing point of the A2Π and B2Σ+ states of ZnH is above the v′ = 2 vibrational level of the A2Π state indicating that the crossings with higher electronic states will have no effect on laser cooling. Hence, we construct a feasible laser-cooling scheme for ZnH using five lasers based on the A2Π1/2 → X2Σ+1/2 transition, which features a large vibrational branching ratio R00 (0.8458), a large number of scattered photons (9.8 × 103) and an extremely short radiative lifetime (64 ns). The present work demonstrates the importance of electronic state crossings and spin-orbit couplings in the study of molecular laser cooling.

Laser cooling molecules to the ultracold regime is the prerequisite for many novel science and technologies. It is desirable to take advantage of theoretical approaches to explore polyatomic molecular candidates, which are capable of being cooled to the ultracold regime. In this work, we explore two polyatomic candidates, CaNC and SrNC, which are suitable for laser cooling. These molecules possess impressively large permanent dipole moments (∼6 Debye), which is preferred for applications using an external electric field. High-level ab initio calculations are carried out to reveal electronic structures of these molecules, and the calculated spectroscopic constants agree very well with the available experimental data. For each molecule, the Franck-Condon factor matrix is calculated and shows a diagonal distribution. The radiative lifetimes for CaNC and SrNC are estimated to be 15.5 and 15.8 ns, respectively. Based upon the features of various electronic states and by choosing suitable spin-orbit states, we construct two feasible laser cooling schemes for the two molecules, each of which allows scattering nearly 10000 photons for direct laser cooling. These indicate that CaNC and SrNC are excellent ultracold polyatomic candidates with strong polarity.

By means of highly accurate ab initio calculations, we identify two excellent ultracold molecular candidates from group VA hydrides. We find that NH and PH are suitable for the production of ultracold molecules, and the feasibility and advantage of two laser cooling schemes are demonstrated, which involve different spin-orbit states (A3Π2 and X3Σ1− ). The internally contracted multireference configuration interaction method is applied in calculations of the six low-lying Λ-S states of NH and PH with the spin-orbit coupling effects included, and excellent agreement is achieved between the computed and experimental spectroscopic data. We find that the locations of crossing point between the A3Π and Σ−5 states of NH and PH are higher than the corresponding v′ = 2 vibrational levels of the A3Π state indicating that the crossings with higher electronic states would not affect laser cooling. Meanwhile, the extremely small vibrational branching loss ratios of the A3Π2 → a1Δ2 transition for NH and PH (NH: 1.81 × 10–8; PH: 1.08 × 10–6) indicate that the a1Δ2 intermediate electronic state will not interfere with the laser cooling. Consequently, we construct feasible laser-cooling schemes for NH and PH using three lasers based on the A3Π2 → X3Σ1− transition, which feature highly diagonal vibrational branching ratio R00 (NH: 0.9952; PH: 0.9977), the large number of scattered photons (NH: 1.04×105; PH: 8.32×106) and very short radiative lifetimes (NH: 474 ns; PH: 526 ns). Our work suggests that feasible laser-cooling schemes could be established for a molecular system with extra electronic states close to those chosen for laser-cooling.

The feasibility of direct laser cooling of SiH, GeH, SnH, and PbH is investigated and assessed based upon first principles. The internally contracted multi-reference configuration interaction method with the Davidson correction is applied. Very good agreement is obtained between our computed spectroscopic constants and the available experimental data. We find that the locations of crossing point between the B2Σ− and A2Δ states have the tendency of moving downwards from CH to SnH relative to the bottom of the corresponding A2Δ potential, which precludes the laser cooling of GeH, SnH, and PbH. By including the spin-orbit coupling effects and on the basis of the A2Δ5/2→X2Π3/2 transition, we propose a feasible laser cooling scheme for SiH using three lasers with wavelengths varying from 400 to 500 nm, which features a very large vibrational branching ratio (0.9954) and a very short radiative lifetime (575 ns). Moreover, similar studies are extended to carbon monosulfide (CS) with a feasible laser cooling scheme proposed. The importance of electronic state crossing in molecular laser cooling is underscored, and our work suggests useful caveats to the choice of promising candidates for producing ultracold molecules.

It is commonly held that van der Waals wells are inevitable in chemical reactions. Here, the authors show that weak van der Waals forces in the entrance channel of a prototypical complex-forming reaction cause a van der Waals saddle instead, with different dynamical effects from a well at low collision energies.

Double proton transfer plays an important role in biology and chemistry, such as with DNA base pairs, proteins and molecular clusters, and direct information about these processes can be obtained from tunneling splittings. Carboxylic acid dimers are prototypes for multiple proton transfer, of which the formic acid dimer is the simplest one. Here, we present efficient quantum dynamics calculations of ground-state and fundamental excitation tunneling splittings in the formic acid dimer and its deuterium isotopologues. These are achieved with a multidimensional scheme developed by us, in which the saddle-point normal coordinates are chosen, the basis functions are customized for the proton transfer process, and the preconditioned inexact spectral transform method is used to solve the resultant eigenvalue problem. Our computational results are in excellent agreement with the most recent experiments (Zhang et al., 2017; Li et al., 2019).

Owing to the exciting potential applications of ultracold atoms and molecules in many fields, developing newcooling schemes has attracted great interests in recent years. Here, we investigate laser cooling of CaBr molecules and design a photonic scheme for the production of ultracold Br atoms using the highly accurate ab initio and dynamical methods. We find that the A²Π1/2(v' = 0) → X²Σ1/2+(v= 0) transition for CaBr features a large vibrational branching ratio, a significant photon-scattering rate, and no intermediate electronic-state interference, indicating that the ultracold CaBr could be produced through a three-laser cooling scheme. Moreover, an efficient four-pulse excitation scheme from the ground rovibrational level of the cooled CaBr molecules is proposed to yield ultracold Br atoms, in which a few spin-orbit excited states are utilized as the intermediate states. The importance of the spin-orbit coupling is underscored in this work. [ABSTRACT FROM AUTHOR]

The reactivity of Fe(super +) toward OCS on both quartet and sextet potential energy surfaces was investigated in this study at the B3LYP/6-311+G(d) level of theory. The studies results indicate that both the C-O and C-S bond activations proceed according to an insertion elimination mechanism would be general for the reaction of first-row transition with small sulfide molecules.

The accurate time-independent quantum dynamics calculations on the ground-state tunneling splitting of malonaldehyde in full dimensionality are reported for the first time. This is achieved with an efficient method developed by us. In our method, the basis functions are customized for the hydrogen transfer process which has the effect of greatly reducing the size of the final Hamiltonian matrix, and the Lanczos method and parallel strategy are used to further overcome the memory and central processing unit time bottlenecks. The obtained ground-state tunneling splitting of 24.5 cm-1 is in excellent agreement with the benchmark value of 23.8 cm-1 computed with the full-dimensional, multi-configurational time-dependent Hartree approach on the same potential energy surface, and we estimate that our reported value has an uncertainty of less than 0.5 cm-1. Moreover, the role of various vibrational modes strongly coupled to the hydrogen transfer process is revealed. [ABSTRACT FROM AUTHOR]

Nine doublet Λ-S states of calcium monochloride (CaCl) are calculated using the internally contracted multireference configuration interaction method with the Davidson correction. Both the core subvalence and spin-orbit coupling effects are taken into account. Laser cooling of CaCl and production of ultracold chlorine atoms are investigated and assessed. Our computed spectroscopic constants and radiative lifetimes match the available experimental data very well. The determined Franck-Condon factors and vibrational branching ratios of the A²π1/2(v′) ← X²Σ+1/2 (v) transition are highly diagonally distributed and the evaluated radiative lifetime for the A²π1/2(v′ = 0) state is 28.2 ns, which is short enough for rapid laser cooling. Subsequently, detection of cold molecules via resonance enhanced multiphoton ionization to determine the final quantum state populations is discussed and the ionization energy calculated. A multi-pulse excitation scheme is proposed for producing ultracold chlorine atoms from zero-energy photodissociation of the cooled CaCl. Our results demonstrate the possibility of producing ultracold CaCl molecules and Cl atoms. [ABSTRACT FROM AUTHOR]

Accurate quantum dynamics calculations for the C(1D) + H2 reaction are performed using a real wave packet approach with full Coriolis coupling. The newly constructed ZMB-a ab initio potential energy surface [Zhang et al., J. Chem. Phys. 140, 234301 (2014)] is used. The integral cross sections (ICSs), differential cross sections (DCSs), and product state distributions are obtained over a wide range of collision energies. In contrast to previous accurate quantum dynamics calculations on the reproducing kernel Hilbert space potential energy surface, the present total ICS is much larger at low collision energies, yielding larger rate coefficients in better agreement with experiment and with slight inverse temperature dependence. Meanwhile, interesting nonstatistical behaviors in the DCSs are revealed. In particular, the DCSs display strong oscillations with the collision energy; forward biased product angular distribution appears when only small J partial wave contributions are included; alternate forward and backward biases emerge with very small increments of collision energy; and the rotational state-resolved DCSs show strong oscillations with the scattering angle. Nevertheless, the total DCSs can be roughly regarded as backward-forward symmetric over the whole energy range and are in reasonably good agreement with the available experimental measurements. [ABSTRACT FROM AUTHOR]

A new global ab initio potential energy surface (called ZMB-a) for the 11A' state of the C(1D)+H2 reactive system has been constructed. This is based upon ab initio calculations using the internally contracted multireference configuration interaction approach with the aug-cc-pVQZ basis set, performed at about 6300 symmetry unique geometries. Accurate analytical fits are generated using many-body expansions with the permutationally invariant polynomials, except that the fit of the deep well region is taken from our previous fit. The ZMB-a surface is unique in the accurate description of the regions around conical intersections (CIs) and of van der Waals (vdW) interactions. The CIs between the 11A' and 21A' states cause two kinds of barriers on the ZMB-a surface: one is in the linear H-CH dissociation direction with a barrier height of 9.07 kcal/mol, which is much higher than those on the surfaces reported before; the other is in the C(1D) collinearly attacking H2 direction with a barrier height of 12.39 kcal/mol. The ZMB-a surface basically reproduces our ab initio calculations in the vdW interaction regions, and supports a linear C-HH vdW complex in the entrance channel, and two vdW complexes in the exit channel, at linear CH-H and HC-H geometries, respectively. [ABSTRACT FROM AUTHOR]

The SiH4+H→SiH3+H2 reaction has been investigated by the quasiclassical trajectory (QCT) method on a recent global ab initio potential energy surface [M. Wang et al., J. Chem. Phys. 124, 234311 (2006)]. The integral cross section as a function of collision energy and thermal rate coefficient for the temperature range of 300–1600 K have been obtained. At the collision energy of 9.41 kcal/mol, product energy distributions and rovibrational populations are explored in detail, and H2 rotational state distributions show a clear evidence of two reaction mechanisms. One is the conventional rebound mechanism and the other is the stripping mechanism similar to what has recently been found in the reaction of CD4+H [J. P. Camden et al., J. Am. Chem. Soc. 127, 11898 (2005)]. The computed rate coefficients with the zero-point energy correction are in good agreement with the available experimental data. [ABSTRACT FROM AUTHOR]

Potential energy surface (PES) intersection seams of two or more electronic states from the 1 1A′, 2 1A′, 3 1A′, 1 1A″, and 2 1A″ states in the C(1D)H2 reactive system are investigated using the internally contracted multireference configuration interaction method and the aug-cc-pVQZ basis set. Intersection seams with energies less than 20 kcal/mol relative to the C(1D)+H2 asymptote are searched systematically, and finally several seam lines (at the linear H–C–H, linear C–H–H, and C2v, geometries, respectively) and a seam surface (at Cs geometries) are discovered and determined. The minimum energy crossing points on these seams are reported and the influences of the PES intersections, in particular, conical intersections, on the CH2 spectroscopy and the C(1D)+H2 reaction dynamics are discussed. In addition, geometries and energies of the 1 1A2 and 1 1B2 states of methylene biradical CH2 are reported in detail for the first time. [ABSTRACT FROM AUTHOR]

A global 12-dimensional ab initio interpolated potential energy surface (PES) for the SiH4+H→SiH3+H2 reaction is presented. The ab initio calculations are based on the unrestricted quadratic configuration interaction treatment with all single and double excitations together with the cc-pVTZ basis set, and the modified Shepard interpolation method of Collins and co-workers [K. C. Thompson et al., J. Chem. Phys. 108, 8302 (1998); M. A. Collins, Theor. Chem. Acc. 108, 313 (2002); R. P. A. Bettens and M. A. Collins, J. Chem. Phys. 111, 816 (1999)] is applied. Using this PES, classical trajectory and variational transition state theory calculations have been carried out, and the computed rate constants are in good agreement with the available experimental data. [ABSTRACT FROM AUTHOR]

The ground state (S0) and the lowest singlet excited state (S1) of a newly synthesized red fluorescent material, 2-[3-(2-{4-[(2-Hydroxy-ethyl)-methyl-amino]-phenyl}-vinyl)-5,5-dimethyl-cyclohex-2-enylidene]-malononitrile (A31), are investigated. The S0 and S1 geometries are optimized at the ab initio Hartree-Fock and the singles configuration interaction (CIS) levels of theory, respectively. The CIS and semiempirical Zerner's Intermediate Neglect of Differential Overlap (ZINDO) methods provide the results for the absorption (S0→S1) and emission (S1→S0) transition energies. The Stokes shifts calculated at the CIS and ZINDO levels of theory are obtained. The absorption spectra in various solvents are calculated using the time-dependent density-functional theory method in combination with the polarized continuum model, which are in very good agreement with our experimental measurements. The solvent effects are discussed. [ABSTRACT FROM AUTHOR]

The unrestricted second-order Moller-Plesset perturbation and complete basis set CBS-4M level methods is used to investigate the reaction of O[sup.3.P] with isobutene {{[CH.sub.3]}[.sub.2)]C=[CH.sub.2]}. The experimental results show that site selectivity of the addition of O[sup.3.P] to the terminal carbon atom of double bond of isobutene is weak.

The reaction of Co(super +) with OCS on both triplet and quintet surfaces is investigated at the B3LYP level of density functional theory with the standard 6-311+G(d) basis set. Calculations indicate that both the C-S and C-O bond activations proceed via an insertion-elimination mechanism.

New ab initio potential energy surfaces (PESs) for the two lowest-lying singlet 11A' and 11A'' electronic states of CH2, coupled by the Renner-Teller (RT) effect and meant for the spectroscopic study, are presented. The surfaces are constructed using a dual-level strategy. The internally contracted multireference configuration interaction calculations with the Davidson correction, using the aug-cc-pVQZ basis set, are employed to obtain 3042 points at the lower level. The core and core-valence correlation effects are taken into account in the ab initio calculations with a modified optimized aug-cc-pCVQZ basis set for the higher-level points. The analytical representations of these PESs, with the inclusion of the nonadiabatic RT terms, are obtained by the nonlinear leastsquares fit of the calculated points to three-body expansion. Quantum dynamical calculations are performed on these PESs, and the computed vibronic energy levels for the two singlet electronic states are in excellent agreement with experiment. [ABSTRACT FROM AUTHOR]

This work elucidates new atomic-level mechanisms that may be common in a range of chemical reactions, and our findings are important for the understanding of the nature of polyatomic abstraction and exchange reactions. A global 12-dimensional ab initio potential energy surface (PES), which describes both H+S1H4 abstraction and exchange reactions is constructed, based on the modified Shepard interpolation method and UCCSD(T)/cc-pVQZ energy calculations at 4,015 geometries. This PES has a classical barrier height of 5.35 kcal/mol for abstraction (our best estimate is 5.35 ± 0.15 kcal/mol from extensive ab initio calculations), and an exothermicityof -13.12 kcal/mol, in excellent agreement with experiment Quasiclassical trajectory calculations on this new PES reveal interesting features of detailed dynamical quantities and underlying new mechanisms. Our calculated product angular distributions for exchange are in the forward hemisphere with a tail sideways, and are attributed to the combination of three mechanisms: inversion, torsion-tilt, and side-inversion. With increase of collision energy our calculated angular distributions for abstraction first peak at backward scattering and then shift toward smaller scattering angles, which is explained by a competition between rebound and stripping mechanisms; here stripping is seen at much lower energies, but is conceptually similar to what was observed in the reaction of H+CD4 by Zare and coworkers [Camden JP, et al. (2005) J Am Chem Soc 127:11898-11899]. Each of these atomic-level mechanisms is confirmed by direct examination of trajectories, and two of them (torsion-tilt and side-inversion) are proposed and designated in this work. [ABSTRACT FROM AUTHOR]

Ab initio calculations on the low-lying electronic states of SiF+ are performed using the internally contracted multireference configuration interaction method with the Davidson correction and entirely uncontracted aug-cc-pV5Z basis set. The effects of spin-orbit coupling are accounted for by the state interaction approach with the full Breit–Pauli Hamiltonian. The entire 23 Ω states generated from the 12 valence Λ–S states, which correlate with the first dissociation channel are studied for the first time. Good agreement is found between the calculated results and the available experimental data. The spin-orbit coupling effects on the potential energy curves and spectroscopic properties are studied. Various curve crossings are revealed, which could lead to the predissociation of the a3Π, A1Π, and (2)3Σ+ states and the predissociation pathways are analyzed based upon the calculated spin-orbit matrix elements. The calculated ionization potentials of the ground-state SiF to a few states of SiF+ are in good agreement with the available experimental measurements. Moreover, the transition dipole moments of the dipole-allowed transitions and the transition properties for the A3Π0+–X1Σ+0+ and B3Π1–X1Σ+0+ transitions are predicted, including the Franck–Condon factors and the radiative lifetimes. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008 [ABSTRACT FROM AUTHOR]

The conception of partial potential energy surface (PPES) is presented in this paper. PPES can be abstracted from complete potential energy surface (CPES), therefore, it can be constructed with ab initio quantum chemical method. For the systems of H+H2→H2+H, I+HI→IH+I and I-+HI→IH+I-, the construction and applications of PPES are proposed as typical examples. It can be seen that the applications of PPES demonstrate remarkable virtues in the analysis of reaction mechanism and the formation of scattering resonance states. [ABSTRACT FROM AUTHOR]

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Two new global ab initio potential energy surfaces (called BW1 and BW2) for the ClH[sub 2] reactive system are presented. These are based on internally contracted multireference configuration interaction calculations using a very large basis set, performed at 1200 geometries. Accurate analytical fits have been generated using the functional form proposed by Aguado and Paniagua. The BW1 surface is based on the original ab initio points. This surface slightly underestimates the dissociation energies of the diatomic fragments and overestimates the barrier height. Therefore, a second surface (BW2) has been computed by scaling the correlation energies at all geometries with a constant factor, which was chosen such that the dissociation energies of HCl and H[sub 2] are reproduced more accurately. The barrier heights for the collinear transition state of the Cl+H[sub 2]→HCl+H reaction are computed to be 8.14 kcal/mol and 7.61 kcal/mol for the BW1 and BW2 surfaces, respectively. To these values the spin-orbit correction of 0.84 kcal/mol has to be added, yielding a best estimate for the true barrier height of 8.45 kcal/mol. In the entrance channel of the Cl+H[sub 2]→HCl+H reaction a T-shaped van der Waals well with a depth of 0.51 kcal/mol is found, while in the exit channel a van der Waals well with a collinear geometry and a depth of 0.45 kcal/mol is predicted. For the H+ClH exchange reaction, which also has a collinear transition state, the barrier heights are computed to be 18.5 kcal/mol and 17.9 kcal/mol for BW1 and BW2, respectively. It is shown that the topology of the new surfaces differs qualitatively from previous semiempirical surfaces, and the implications on the dynamics of the H[sub 2]+Cl reaction are discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

New ab initio potential energy surfaces (PESs) for the two lowest-lying singlet 11A' and 11A'' electronic states of CH2, coupled by the Renner-Teller (RT) effect and meant for the spectroscopic study, are presented. The surfaces are constructed using a dual-level strategy. The internally contracted multireference configuration interaction calculations with the Davidson correction, using the aug-cc-pVQZ basis set, are employed to obtain 3042 points at the lower level. The core and core-valence correlation effects are taken into account in the ab initio calculations with a modified optimized aug-cc-pCVQZ basis set for the higher-level points. The analytical representations of these PESs, with the inclusion of the nonadiabatic RT terms, are obtained by the nonlinear leastsquares fit of the calculated points to three-body expansion. Quantum dynamical calculations are performed on these PESs, and the computed vibronic energy levels for the two singlet electronic states are in excellent agreement with experiment. [ABSTRACT FROM AUTHOR]