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The intriguing ability of C -phenyl-nitrilimine to co-exist as allenic and propargylic bond-shift isomers motivated us to investigate how substituents in the phenyl ring influence this behavior. Building on our previous work on the meta - and para -OH substitution, here we extended this investigation to explore the effect of the NH 2 substitution. For this purpose, C -(4-aminophenyl)- and C -(3-aminophenyl)-nitrilimines were photogenerated in an argon matrix at 15 K by narrowband UV-light irradiation (λ = 230 nm) of 5-(4-aminophenyl)- and 5-(3-aminophenyl)-tetrazole, respectively. The produced nitrilimines were further photoisomerized to carbodiimides via 1 H -diazirines by irradiations at longer wavelengths (λ = 380 or 330 nm). Combining IR spectroscopic measurements and DFT calculations, it was found that the para -NH 2 -substituted nitrilimine exists as a single isomeric structure with a predominant allenic character. In contrast, the meta -NH 2 -substituted nitrilimine co-exists as two bond-shift isomers characterized by propargylic and allenic structures. To gain further understanding of the effects of phenyl substitution on the bond-shift isomerism of the nitrilimine fragment, we compared geometric and charge distribution data derived from theoretical calculations performed for C -phenyl-nitrilimine with those performed for the derivatives resulting from NH 2 (electron-donating group) and NO 2 (electron-withdrawing group) phenyl substitutions.

Monomers of meta -fluorophenol ( m FP) were trapped from the gas phase into cryogenic argon and nitrogen matrices. The estimated relative energies of the two conformers are very close, and in the gas phase they have nearly equal populations. Due to the similarity of their structures (they only differ in the orientation of the OH group), the two conformers have also similar predicted vibrational signatures, which makes the vibrational characterization of the individual rotamers challenging. In the present work, it has been established that in an argon matrix only the most stable trans conformer of m FP exists (the OH group pointing away from the fluorine atom). On the other hand, the IR spectrum of m FP in a nitrogen matrix testifies to the simultaneous presence in this matrix of both the trans conformer and of the higher-energy cis conformer (the OH group pointing toward the fluorine atom), which is stabilized by interaction with the matrix gas host. We found that the exposition of the cryogenic N 2 matrix to the Globar source of the infrared spectrometer affects the conformational populations. By collecting experimental spectra, either in the full mid-infrared range or only in the range below 2200 cm -1 , we were able to reliably distinguish two sets of experimental bands originating from individual conformers. A comparison of the two sets of experimental bands with computed infrared spectra of the conformers allowed, for the first time, the unequivocal vibrational identification of each of them. The joint implementation of computational vibrational spectroscopy and matrix-isolation infrared spectroscopy proved to be a very accurate method of structural analysis. Some mechanistic insights into conformational isomerism (the quantum tunneling of hydrogen atom and vibrationally-induced conformational transformations) have been addressed. Finally, we also subjected matrix-isolated m FP to irradiations with UV light, and the phototransformations observed in these experiments are also described.

In this work, monomers of carvacrol (5-isopropyl-2-methylphenol), a natural monoterpene exhibiting wide range bioactivity, were trapped in a cryogenic argon matrix and characterized by infrared spectroscopy, while quantum chemical calculations at the B3LYP and MP2 levels were employed to characterize the conformational landscape of the isolated molecule. Four conformers have been localized on the potential energy surface, and the factors accounting for their relative stability were analyzed. The two most stable conformers of carvacrol, differing in the relative orientation of the isopropyl group and both having the OH group pointing away from the vicinal methyl fragment, were identified in the cryomatrix for the first time. The individual spectral signatures of the two conformers were distinguished based on the change in their relative abundance induced by exposing the matrix to broadband infrared light. Matrix-isolated carvacrol was also irradiated with broadband UV light (λ > 200 nm), which resulted in the cleavage of the OH group. Recombination of the released H atom at the ortho- or para-position of the ring resulted in the formation of alkyl-substituted cyclohexadienones. These were found to undergo subsequent valence and open-ring isomerizations, leading, respectively, to the formation of a Dewar isomer and open-chain conjugated ketenes. Decarbonylation of the photoproducts was also observed for longer irradiation times. A mechanistic analysis of the observed photochemical transformations is presented.

Monomers of 4-methoxyindole and 5-methoxyindole trapped in low-temperature xenon matrices (15-16 K) were characterized by IR spectroscopy, in separate experiments. Each compound was shown to adopt the most stable 1H-tautomeric form. The photochemistry of the matrix-isolated compounds was then investigated by exciting the matrices with narrowband UV light with λ ≤ 305 nm. Two main photoproducts, similar for each compound, have been detected: (1) 4-methoxy- or 5-methoxy-indolyl radical, resulting from cleavage of the N-H bond; (2) 3H-tautomers (4-methoxy- or 5-methoxy-) with the released hydrogen atom reconnected at the C3 ring carbon atom. The presence of the two types of photoproducts in the UV-irradiated matrices was confirmed by comparison of their B3LYP/6-311++G(d,p) calculated IR spectra with the experimental spectra emerging upon the irradiations. The mechanism of the observed phototransformations was elucidated by Natural Bond Orbital and Natural Resonance Theory computations on the methoxy-substituted indolyl radicals resulting from the N-H bond cleavage. The highest natural atomic spin densities were predicted at the C3 and N1 positions of the indolyl ring, corresponding to a predominance of the resonance structures with the radical centres located at these two atoms. As a whole, the obtained experimental and theoretical data allowed establishing a general pattern for the photochemistry of methoxyindoles under matrix-isolation conditions.

Three low-energy isomers of 9-methylguanine, the amino-oxo (AO) form and two amino-hydroxy (AH1 and AH2) conformers, were trapped from the gas phase into low-temperature argon matrices. The AH1 and AH2 isomers, differing in the orientation of the OH group, were found to transform into each other upon excitation with near-IR light. The population of the AO form of the compound was not changed upon any near-IR irradiation of the matrix samples. Using monochromatic near-IR light, generated by a frequency-tunable laser source, it was possible to selectively induce the AH1 → AH2 or AH2 → AH1 conversion. Photoreversibility of this conformational transformation was then demonstrated. Exposure of matrix-isolated monomers of 9-methylguanine to broadband near-IR light also led to conformational conversions within the amino-hydroxy tautomeric form; the final stage of this process was always the same photostationary state independent of the initial ratio of AH1 and AH2 populations. Spontaneous conformational conversion, transforming the higher-energy AH2 form into the lower-energy AH1 isomer, was observed for matrix-isolated monomers of 9-methylguanine kept in the dark. The mechanism of this process must rely on quantum tunneling of the light hydrogen atom. Irradiation of matrix-isolated 9-methylguanine with UV laser light at λ = 288 or 285 nm led to a substantial consumption of the two AH forms, while the amount of AO isomer remained unchanged. On the other hand, a decrease in the population of the AO isomer occurred upon excitations at shorter wavelengths, λ = 280 or 275 nm. The spectral changes observed after UV-irradiation suggest the generation (and stabilization in the matrix) of a radical species, resulting from the photocleavage of the O-H or N1-H bonds, in the AH or AO isomer, respectively.

The conformers of 2-cyanophenol (2CP) and their interconversions were studied by infrared (IR) spectroscopy after trapping the monomers of the gaseous compound into low-temperature (15 K) argon (Ar) and nitrogen (N 2 ) matrixes. To assist in the interpretation of the experimental results, B3LYP, MP2, and QCISD electronic structure calculations were carried out for the 2CP molecule. Two planar conformers, cis and trans (orientation of OH with respect to the cyano group), are predicted with gas-phase populations at the sublimation temperature of ∼98 and ∼2%, respectively. The most stable form (cis) was experimentally identified in both cryomatrixes, whereas the less stable one (trans) was not detected in Ar but could be observed in the N 2 matrix with an abundance of ∼15%. Selective and bidirectional conversion between the two identified conformers was achieved upon irradiating the compound trapped in N 2 matrix with near-infrared (NIR) laser light tuned at the wavenumbers of the 2ν(OH) transitions of the respective conformers. The conformational composition of 2CP was also found to be affected by the broad-band IR radiation emitted by the spectrometer source. This effect could be suppressed, partially or completely, by using different long-pass IR filters, with cutoff values of approximately 2200, 1590, and 1170 cm -1 . The observed conformational changes are rationalized in terms of a competition between the over-the-barrier (light-induced) and through-the-barrier (hydrogen atom tunneling) effects. Very interestingly, both effects occur on the same time scale.

Conformers and near-IR-induced conformational transformations were studied for monomers of glycolamide isolated in low-temperature matrixes. Two conformational isomers of the compound, Tt and Cc, were trapped from the gas phase into solid Ar matrixes. Selective near-IR excitation of glycolamide molecules adopting the Tt form led to the Tt → Cc conformational conversion. Analogously, selective near-IR excitation of Cc conformers resulted in the Cc → Tt transformation. Monochromatic near-IR light, generated by frequency-tunable laser sources, was used for irradiation of matrix-isolated monomers. Near-IR-induced Tt → Cc and Cc → Tt conformational transformations occurred upon excitation of 2νOH, 2ν a NH 2 , and 2ν s NH 2 overtones, as well as upon excitation of ν a NH 2 + ν s NH 2 combination modes. In spite of the structural similarity of glycolamide and N-hydroxyurea, no conformational conversions were observed for monomers of the latter compound isolated in Ar matrixes and excited with near-IR light. The comparison of the effects of near-IR excitations of glycolamide and N-hydroxyurea demonstrates that transformations involving rotation of molecular fragments around a single C-C bond occur much easier than transformations involving rotation of the fragments around a C-N bond. The efficiency of the latter conversions is extremely low.

We apply interchangeable vibrational antennas (OH or NH2 group) to achieve unprecedented conformational control over the heavy aldehyde fragment in 2-formyl-2H-azirine. The two aldehyde conformers were manipulated bi-directionally, using selective vibrational excitation with narrowband near-infrared (NIR) light tuned at the wavenumbers corresponding to OH and NH2 stretching overtones and combination modes.

Photochemical transformations were studied for monomers of indole and 3-formylindole isolated in low-temperature noble-gas matrices. Upon UV (λ > 270 nm) irradiation of indole trapped in argon and neon matrices, the initial 1H-form of the compound converted into the 3H-tautomer. Alongside this photoinduced hydrogen-atom transfer, an indolyl radical was also generated by photodetachment of the hydrogen atom from the N1-H bond. Excitation of 3-formylindole isolated in an argon matrix with UV (λ > 335 nm) light led to interconversion between the two conformers of the 1H-tautomer, differing from each other in the orientation of the formyl group (cis or trans). Parallel to this conformational phototransformation, the 3H-form of the compound was generated in the 1H → 3H phototautomeric conversion. The photoproducts emerging upon UV irradiation of indole and 3-formylindole were identified by comparison of their infrared spectra with the spectra calculated for candidate structures.

The molecule of 5-methoxyindole (5MOI) may adopt two conformational states, syn and anti, with respect to the relative orientation of the NH and OCH 3 groups. The structure of monomeric 5MOI was characterized spectroscopically, in mid- and near-infrared domains. The conformational composition of 5MOI could be controlled in three different ways. Thermally, two conformers of 5MOI could be trapped in xenon matrixes at 16 K. Upon annealing the xenon matrix to temperatures about 30-40 K, the higher-energy syn form converted to the ground-state anti conformer. Vibrational excitations in the near-infrared domain, at the frequency of the first NH stretching overtone, 6853 cm -1 , afforded the inverse conformational transformation, and a part of the anti conformer was upconverted to the syn form. Electronic excitations in the UV domain, at 315-310 nm, resulted in a total consumption of the syn form again, in favor of anti. Upon further irradiations at 308 nm, a partial repopulation of the syn form, at the expense of anti, was observed. We propose a mechanistic explanation of the observed transformations, which is based on computations of the vibrational spectra of the two conformers and also on computations of the ground state S 0 and the first excited state S 1 potential energy surfaces along the coordinate for conformational isomerization. The highlights of the present work are the first experimental observation of the minor syn conformer of 5MOI, evidence of the long-range vibrational energy transfer resulting in conformational isomerization upon excitation of the NH stretching overtone, and the possibility of partial conformational control of 5MOI by using electronic excitations.

Conformational changes induced thermally or upon infrared excitation of matrix-isolated 6-methoxyindole were investigated. Narrowband near-infrared excitation of the first overtone of the N-H stretching vibration of each one of the two identified conformers is found to induce a selective large-scale conversion of the pumped conformer into the other one. This easily controllable bidirectional process consists in the intramolecular reorientation of the methoxy group and allowed a full assignment of the infrared spectra of the two conformers. Matrices with different conformational compositions prepared by narrow-band irradiations were subsequently used to investigate the effects of both thermal and broadband infrared excitations on the conformational mixtures. Particular attention is given to the influence of the matrix medium (Ar vs. Xe) and conformational effects of exposition of the sample to the spectrometer light source during the measurements.

Here we describe an unprecedented reversible interconversion between two conformational states, accomplished by selective vibrational excitation of a bond remotely located in relation to the isomerizing fragment. In contrast to previous studies reporting conformational changes via vibrational excitation of a nearby OH group, in this study the transformations were successfully achieved by excitation of a distant NH stretching coordinate. The syn and anti forms of monomeric 6-methoxyindole, which differ in the orientation of the methoxy group, were trapped in low-temperature inert matrixes and characterized spectroscopically. These forms could be selectively shifted in both directions by using near-IR excitations tuned at the frequency of the first NH stretching overtone. The observed isomerization proves the possibility of efficient vibrational relaxation to carry the energy deposited at the NH stretching coordinate into the reactive C-O torsional mode localized on the methoxy group four bonds away from the excited NH moiety.

The conformational space of 3,4-dihydroxyphenylacetic acid (DOPAC), an important dopamine metabolite, has been investigated by quantum chemical methods (B3LYP and MP2, with the 6-311++G(d,p) basis set) and matrix-isolation infrared spectroscopy. Detailed analysis of the calculated potential energy surfaces of the molecule led to identification of thirteen unique conformers, all of them showing the acetic acid side chain out of the aromatic ring plane by 60-95°. According to the calculated Gibbs energies, the five lowest energy conformers make up 99.7% of the conformational mixture at 298.15K, exhibiting individual populations falling between 16% and 24%. The main conformational trends of this molecule were interpreted on the grounds of a thorough analysis of the structural parameters and by the application of the Natural Bond Orbital theory. The role of the intramolecular interactions on the relative stability and structure of the conformers was also investigated. The infrared spectrum of DOPAC was registered after isolation of its monomers in argon and xenon matrices. Only one of DOPAC forms populated in the gas phase could be trapped in both matrix gases. This result is in agreement with the predicted low energy barriers for conformational isomerization and is also supported by annealing experiments. The spectra of matrix-isolated model compounds, phenylacetic acid and catechol, were studied under the same experimental conditions. These data were used as references and assisted in the interpretation of the results obtained for DOPAC., (Copyright © 2014 Elsevier B.V. All rights reserved.)