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1.
Phys Chem Chem Phys ; 26(9): 7865-7876, 2024 Feb 28.
Article in English | MEDLINE | ID: mdl-38376442

ABSTRACT

Carotenoids are very effectively delivered by albumin to adipocytes. The uptake of carotenoids to the cells occurs in the form of self-aggregates that localize in the vicinity of the adipocyte membrane, as shown by high spatial resolution Raman spectroscopy. The binding of carotenoids to albumin and the mechanism of their transport were elucidated with the help of chiroptical spectroscopies, in tandem with molecular docking and molecular dynamics simulations. In particular, apart from the recognized high affinity pocket of albumin that binds a carotenoid monomer in domain I, we have identified a hydrophobic periphery area in domain IIIB that loosely bounds the self-aggregated carotenoid in aqueous media and enables its easy detachment in hydrophobic environments. This explains the effectiveness of albumins as nanocarriers of carotenoids to adipocytes in vitro.


Subject(s)
Albumins , Carotenoids , Carotenoids/chemistry , Molecular Docking Simulation , Biological Transport , Adipocytes/metabolism , Spectrum Analysis, Raman/methods
2.
J Org Chem ; 88(5): 2884-2897, 2023 Mar 03.
Article in English | MEDLINE | ID: mdl-36795993

ABSTRACT

Monomers of benzimidazole trapped in an argon matrix at 15 K were characterized by vibrational spectroscopy and identified as 1H-tautomers exclusively. The photochemistry of matrix-isolated 1H-benzimidazole was induced by excitations with a frequency-tunable narrowband UV light and followed spectroscopically. Hitherto unobserved photoproducts were identified as 4H- and 6H-tautomers. Simultaneously, a family of photoproducts bearing the isocyano moiety was identified. Thereby, the photochemistry of benzimidazole was hypothesized to follow two reaction pathways: the fixed-ring and the ring-opening isomerizations. The former reaction channel results in the cleavage of the NH bond and formation of a benzimidazolyl radical and an H-atom. The latter reaction channel involves the cleavage of the five-membered ring and concomitant shift of the H-atom from the CH bond of the imidazole moiety to the neighboring NH group, leading to 2-isocyanoaniline and subsequently to the isocyanoanilinyl radical. The mechanistic analysis of the observed photochemistry suggests that detached H-atoms, in both cases, recombine with the benzimidazolyl or isocyanoanilinyl radicals, predominantly at the positions with the largest spin density (revealed using the natural bond analysis computations). The photochemistry of benzimidazole therefore occupies an intermediate position between the earlier studied prototype cases of indole and benzoxazole, which exhibit exclusively the fixed-ring and the ring-opening photochemistries, respectively.

3.
Molecules ; 27(23)2022 Nov 26.
Article in English | MEDLINE | ID: mdl-36500356

ABSTRACT

Monomers of meta-fluorophenol (mFP) 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 mFP exists (the OH group pointing away from the fluorine atom). On the other hand, the IR spectrum of mFP 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 N2 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 mFP to irradiations with UV light, and the phototransformations observed in these experiments are also described.


Subject(s)
Nitrogen , Ultraviolet Rays , Argon/chemistry , Nitrogen/chemistry , Molecular Conformation , Spectrophotometry, Infrared
4.
J Org Chem ; 86(9): 6126-6137, 2021 05 07.
Article in English | MEDLINE | ID: mdl-33872502

ABSTRACT

The monomers of 1,3-benzoxazole isolated in a cryogenic argon matrix were characterized by infrared spectroscopy. The photochemistry of matrix-isolated 1,3-benzoxazole, induced by excitation with a frequency-tunable narrowband UV light, was investigated. Irradiation at 233 nm resulted in a nearly quantitative conversion of 1,3-benzoxazole into 2-isocyanophenol. The individual photochemical behavior of the in situ produced 2-isocyanophenol was studied upon excitations at 290 nm, where 1,3-benzoxazole does not react. The photochemistry of isomeric matrix-isolated 2-cyanophenol was also studied. The photoreactions of 2-substituted (cyano- or isocyano-) phenols were found to have many similarities: (i) OH bond cleavage, yielding a 2-substituted (cyano- or isocyano-) phenoxyl radical and an H-atom, (ii) recombination of the detached H-atom, resulting in an oxo tautomer, and (iii) decomposition leading to fulvenone, together with HCN and HNC. In another photoprocess, 2-cyanophenol undergoes a [1,5] H-shift from the hydroxyl group to the cyano group yielding isomeric ketenimine. The analogous [1,5] H-shift from the hydroxyl group to the isocyano group must have also occurred in 2-isocyanophenol; however, the resulting nitrile ylide isomer is kinetically unstable and collapses to benzoxazole. All photoproducts were characterized by comparing their observed infrared spectra with those computed at the B3LYP/6-311++G(d,p) level. The mechanistic analysis of the photochemistry occurring in the family of the title compounds is presented.

5.
J Phys Chem A ; 125(37): 8215-8229, 2021 Sep 23.
Article in English | MEDLINE | ID: mdl-34506137

ABSTRACT

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.


Subject(s)
Argon/chemistry , Cymenes/isolation & purification , Temperature , Ultraviolet Rays , Cymenes/chemistry , Infrared Rays , Molecular Conformation
6.
J Phys Chem A ; 125(29): 6394-6403, 2021 Jul 29.
Article in English | MEDLINE | ID: mdl-34275275

ABSTRACT

Thiotropolone isolated in argon and xenon matrices (as monomers) or in a neat solid (as the crystalline or amorphous state) at low temperature was found to exist only in the thione-enol form. Visible light irradiation (λ ≥ 400 nm) leads to thione-enol → thiol-keto tautomerization in matrices and under neat solid conditions at 15 K. The assignment of the IR spectra of the two thiotropolone tautomers (thione-enol and thiol-keto) was carried out with the support of B3LYP/6-311+G(2d,p) computations. The thiol-keto form generated in situ in a neat solid was found to tautomerize back to the thione-enol upon annealing up to 100 K. Gaussian-4 (G4) computations estimate that such a tautomerization process has an energy barrier of ∼25 kJ mol-1, which is consistent with the observations. Moreover, it was found that narrowband IR irradiation of the thiol-keto form in a neat solid, at the frequency of its CH stretching overtones/combination modes, also induces tautomerization to the thione-enol form. Such a result constitutes an important demonstration of vibrationally induced chemistry under neat solid conditions.

7.
J Phys Chem A ; 124(49): 10277-10287, 2020 Dec 10.
Article in English | MEDLINE | ID: mdl-33245233

ABSTRACT

The conformational behavior of carboxylic acids has attracted considerable attention, as it can be used as a gateway for the study of more complex phenomena. Here, we present an experimental and computational study of pyrrole-2-carboxylic acid (PCA) conformational space and the vibrational characterization of the compound by infrared spectroscopy. The possibility of promoting conformational transformations using selective vibrational excitation of the 2ν(OH) and 2ν(NH) stretching overtones is explored. Two conformers, exhibiting the cis configuration of the COOH group (O═C-O-H dihedral angle near 0°) and differing by the orientation of the carboxylic group with respect to the pyrrole ring (i.e., showing either a cis or a trans NCC═O arrangement), were found to coexist initially for the compound isolated in a cryogenic nitrogen matrix, in an 86:14 ratio, and were characterized by infrared spectroscopy. A third conformer, with the COOH group in the trans configuration, was produced, in situ, by narrowband near-infrared (NIR) excitation of the most stable PCA form (with a cis NCC═O moiety). The photogenerated PCA conformer was found to decay back to the most stable PCA form, by H-atom quantum mechanical tunneling, with a characteristic half-life time of ∼10 min in the nitrogen matrix at 10 K. Tunneling rates were theoretically estimated and compared for the observed isomerization of pyrrole-2-carboxylic acid and for the structurally similar furan-2-carboxylic acid. This comparison showcases the effect of small modifications in the potential energy surface and the implications of quantum tunneling for the stability of short-living species.

8.
J Am Chem Soc ; 141(36): 14340-14348, 2019 09 11.
Article in English | MEDLINE | ID: mdl-31423776

ABSTRACT

Quantum mechanical tunneling (QMT) of heavy atoms like carbon or nitrogen has been considered very unlikely for the longest time, but recent evidence suggests that heavy-atom QMT does occur more frequently than typically assumed. Here we demonstrate that carbon vs nitrogen heavy-atom QMT can even be competitive leading to two different products originating from the same starting material. Amino-substituted benzazirine was generated in solid argon (3-18 K) and found to decay spontaneously in the dark, with a half-life of 210 min, to p-aminophenylnitrene and amino-substituted ketenimine. The reaction rate is independent of the cryogenic temperature, in contradiction to the rules inferred from classical transition state theory. Quantum chemical computations confirm the existence of two competitive carbon vs nitrogen QMT reaction pathways. This discovery emphasizes the quantum nature of atoms and molecules, thereby enabling a much higher level of control and a deeper understanding of the factors that govern chemical reactivity.

9.
Phys Chem Chem Phys ; 21(45): 24993-25001, 2019 Dec 07.
Article in English | MEDLINE | ID: mdl-31710324

ABSTRACT

Vibrational excitation using frequency-tunable IR laser light has been developed as a powerful tool for selective manipulation of molecular conformations. In this methodology, vibrational excitation has been typically applied to the first stretching overtones (∼80 kJ mol-1) but also to the fundamental modes (∼40 kJ mol-1). Here, we demonstrate that selective conformational isomerizations are also achieved using excitation to second stretching overtones (∼120 kJ mol-1). The extremely weak absorptions of the second stretching overtones of molecules isolated in low-temperature matrices were measured for the first time; here using three prototype molecules: hydroxyacetone (HA), glycolic acid (GAc) and glycolamide (GAm). Benchmarking of computed anharmonic IR spectra showed that the B3LYP/SNSD method provides the best agreement with experimental frequencies of the ν(OH), 2ν(OH) and 3ν(OH) modes for the studied molecules in argon matrices. Selective irradiation at the 3ν(OH) frequencies (9850-10 500 cm-1) of HA, GAc and GAm monomers in argon matrices at 15 K successfully triggers their conformational isomerization. These results open the door to extend control over conformations separated by higher barriers and to induce other transformations not energetically accessible by excitation to the fundamental or first stretching overtone modes.

10.
Phys Chem Chem Phys ; 21(31): 17063-17071, 2019 Aug 21.
Article in English | MEDLINE | ID: mdl-31276124

ABSTRACT

Rotamerization of a hydroxyl (O-H) group by tunneling is well-known and has been extensively studied. On the other hand, similar tunneling processes for the thiol (S-H) group have not been reported yet. In this work, the imino-thiol forms of thioacetamide were studied in cryogenic matrices (Ar, Xe) after UV-irradiation of the common amino-thione form of the compound. Four different imino-thiol forms were generated, corresponding to the cis or trans thiol (C/T) conformers of the two imino isomers (syn and anti; s/a). Noteworthy, the syn-cis (sC) imino-thiol form was found to convert spontaneously to the syn-trans (sT) form (with a half-life of 80 min), in a process whose reaction rate is independent of the temperature (i.e., at 11 or 20 K). Such conformational transformation represents the first experimental observation of an S-H rotamerization occurring by tunneling. Computations based on the Wentzel-Kramers-Brillouin formalism predict a tunneling half-life for the S-H rotamerization of syn-imino sC to sT on the time scale of minutes, in agreement with the experimental observations.

11.
Phys Chem Chem Phys ; 21(45): 24935-24949, 2019 Dec 07.
Article in English | MEDLINE | ID: mdl-31701978

ABSTRACT

An imino group was used for the first time as a vibrational antenna to manipulate molecular conformations. Imino-thiol isomers of thioacetamide were generated upon UV-irradiation of its amino-thione tautomer isolated in argon matrices at 11 K. Selective and reversible conformational isomerizations were induced by narrowband near-IR irradiation tuned at the frequencies of the 2ν(NH) first stretching overtone of each imino-thiol isomer. The conformational isomerization concerns the change in the orientation of a remote -SH group, while the orientation of the imino (C[double bond, length as m-dash]NH) group remains the same. Supported by quantum chemical anharmonic computations, this allowed for a reliable, isomer-selective vibrational assignment of the four imino-thiol isomers extending now over the full mid-IR and near-IR ranges. It was found that the experimental IR intensities of the 2ν(NH) first stretching overtones (computed 4-5 km mol-1) of the imino-thiol forms are comparable to those of the ν(NH) stretching fundamentals (computed 2-4 km mol-1). This is the first time such a phenomenon is reported for an imine molecule. The kinetics of conformational isomerization was monitored in situ, indicating that the irradiation-induced processes are significantly faster than the tunneling-driven spontaneous cis-trans rotamerization of the -SH group. Quantum yields for the rotamerizations of the -SH group resulting from the vibrational excitation of a remote -NH group were estimated and found to be comparable to those observed for matrix-isolated carboxylic acids and amino acids, where conformational changes of the -OH group were induced by the direct vibrational excitation of 2ν(OH) first stretching overtones.

12.
Phys Chem Chem Phys ; 21(41): 22857-22868, 2019 Oct 24.
Article in English | MEDLINE | ID: mdl-31599896

ABSTRACT

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.


Subject(s)
Argon/chemistry , Cold Temperature , Guanine/analogs & derivatives , Infrared Rays , Ultraviolet Rays , Guanine/chemistry , Isomerism , Molecular Conformation/radiation effects
13.
J Phys Chem A ; 123(17): 3831-3839, 2019 May 02.
Article in English | MEDLINE | ID: mdl-30943718

ABSTRACT

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, 2νaNH2, and 2νsNH2 overtones, as well as upon excitation of νaNH2 + νsNH2 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.

14.
J Chem Phys ; 149(10): 104301, 2018 Sep 14.
Article in English | MEDLINE | ID: mdl-30218999

ABSTRACT

UV-induced transformations were studied for monomers of 6-azacytosine isolated in low-temperature Ar matrices. In contrast to cytosine, where the amino-hydroxy (AH) tautomer is the lowest-energy form, the amino-oxo (AO) and imino-oxo (IO) isomers of 6-azacytosine were found to be the most stable and most populated. Due to the high relative energy of the AH tautomer of 6-azacytosine, this form is not populated in low-temperature matrices after their formation and prior to any irradiation. Excitation of 6-azacytosine monomers with UV light from the 328-300 nm range led to structural transformations of AO and IO forms. The initially most populated AO tautomer was observed either to convert, in a phototautomeric reaction, into the AH product or to undergo photodecarbonylation to yield 4-amino-1,2,3-(2H)-triazole. The relative efficiencies of the two processes depend on the wavelength and on the pulsed or continuous-wave character of the UV light used for excitation. For the IO tautomer of 6-azacytosine, the excitation with UV 328-300 nm light induced the photoconversion of the initially more populated anti IO1 isomer into the syn IO2 form. This transformation was found to be partially photoreversible.

15.
J Am Chem Soc ; 139(48): 17649-17659, 2017 12 06.
Article in English | MEDLINE | ID: mdl-29112415

ABSTRACT

The slippery potential energy surface of aryl nitrenes has revealed unexpected and fascinating reactions. To explore such a challenging surface, one powerful approach is to use a combination of a cryogenic matrix environment and a tunable narrowband radiation source. In this way, we discovered the heavy-atom tunneling reaction involving spontaneous ring expansion of a fused-ring benzazirine into a seven-membered ring cyclic ketenimine. The benzazirine was generated in situ by the photochemistry of protium and deuterated triplet 2-formylphenylnitrene isolated in an argon matrix. The ring-expansion reaction takes place at 10 K with a rate constant of ∼7.4 × 10-7 s-1, despite an estimated activation barrier of 7.5 kcal mol-1. Moreover, it shows only a marginal increase in the rate upon increase of the absolute temperature by a factor of 2. Computed rate constants with and without tunneling confirm that the reaction can only occur by a tunneling process from the ground state at cryogenic conditions. It was also found that the ring-expansion reaction rate is more than 1 order of magnitude faster when the sample is exposed to broadband IR radiation.

16.
Phys Chem Chem Phys ; 19(18): 11447-11454, 2017 May 10.
Article in English | MEDLINE | ID: mdl-28425538

ABSTRACT

Photochemical transformations were investigated for monomers of 7-azaindole isolated in low-temperature Ar and normal-H2 (n-H2) matrices. The most stable N1H tautomer was the only form of the compound populated in Ar and n-H2 matrices before any irradiation. Upon exposure of Ar matrices to UV (λ > 270 nm) light, two higher-energy tautomers N7H and C3H were photoproduced. Additionally, spectral signatures of the photogenerated 7-azaindolyl radical were also found. All of these photoproducts were experimentally observed for the first time. So far, the N7H tautomer had been known only as a transient species, appearing upon relaxation of photoexcited hydrogen-bonded dimers or complexes. For 7-azaindole isolated in an n-H2 matrix and irradiated at λ > 270 nm, only the C3H tautomer and the 7-azaindolyl radical were photogenerated, whereas the N7H tautomer was not photoproduced at all. The drastic dependence of photogeneration of the N7H form on the matrix environment (solid Ar or solid n-H2) is as a characteristic feature of a specific class of UV-induced hydrogen-atom-transfer processes occurring in matrix-isolated heterocycles.

17.
J Chem Phys ; 147(19): 194304, 2017 Nov 21.
Article in English | MEDLINE | ID: mdl-29166121

ABSTRACT

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.

18.
J Am Chem Soc ; 138(47): 15287-15290, 2016 11 30.
Article in English | MEDLINE | ID: mdl-27682681

ABSTRACT

Triplet 2-formyl phenylnitrene was generated by photolysis of 2-formyl phenylazide isolated in Ar, Kr, and Xe matrixes and characterized by IR, UV-vis, and EPR spectroscopies. Upon generation at 10 K, the triplet nitrene spontaneously rearranges in the dark to singlet 6-imino-2,4-cyclohexadien-1-ketene on the time scale of several hours. The intramolecular [1,4] H atom shift from the nitrene to the imino ketene occurs by tunneling, on the triplet manifold, followed by intersystem crossing. This case constitutes the first direct evidence of a tunneling reaction involving a nitrene.

19.
Phys Chem Chem Phys ; 18(25): 16802-11, 2016 Jun 22.
Article in English | MEDLINE | ID: mdl-27279432

ABSTRACT

2,2'-Dihydroxyazobenzene (DAB), 2,2'-azotoluene (AT) and azobenzene (AB) were isolated in argon and xenon matrices and their molecular structures and photochemical transformations were characterized by infrared spectroscopy and theoretical calculations. All these compounds can adopt the E and Z isomeric forms around the central CNNC moiety, which can be enriched by several conformational and tautomeric modifications for DAB and AT. A number of DAB and AT isomeric forms were identified for the first time. For DAB, the E azo-enol isomer with two intramolecular six-membered quasi-rings formed via OHN hydrogen bonds was found after deposition. Irradiation with UV light generated a different E azo-enol form with two intramolecular H-bonded five-membered quasi-rings. Phototransformation was shown to be reversible and the forms could be interconverted by irradiation at different wavelengths. The isomerization between these two forms constitutes a direct experimental observation of an E → E isomerization in azobenzene-type molecules. Further irradiation generated a form(s) bearing both OH and NH groups. For AT, two E isomers with the CH3 groups forming five-membered and five/six-membered quasi-rings with the azo group were observed in the as-deposited matrices. Irradiation of AT with UV light generated a Z form that can be converted back to the E form at different irradiation wavelengths. E-AB was deposited in a xenon matrix and both E → Z and Z → E phototransformations were observed (contrary to what was previously reported in an argon matrix where only the Z → E conversion occurred). AB photoisomerization becomes more pronounced at elevated temperatures, thus indicating that the matrix effects responsible for hindering the AB photoisomerization are essentially due to steric restrictions. The different photoisomerization channels observed for these compounds are discussed in terms of a small-amplitude pedal motion.

20.
J Phys Chem A ; 120(17): 2647-56, 2016 05 05.
Article in English | MEDLINE | ID: mdl-27070389

ABSTRACT

Conformational transformations were investigated for molecules of kojic acid trapped in low-temperature argon and nitrogen matrixes. Two conformers, differing from each other by 120° rotation of the hydroxymethyl (-CH2OH) moiety, were found to be populated in freshly deposited matrixes, prior to any irradiation. Matrixes containing isolated monomers of kojic acid were irradiated with narrowband, tunable near-infrared (near-IR) laser light. Excitations at wavenumbers corresponding to the overtone of the stretching vibration of the OH bond of the hydroxymethyl group led to conversion of one of the observed conformers into another. The direction of this conformational transformation depended on the wavenumber (within the 7126-7115 cm(-1) range) used for irradiation. The same conformational photoconversion was also observed to occur upon narrowband irradiation at much lower wavenumbers (from the 6468-6447 cm(-1) range). Near-IR light from this range selectively excites overtone vibrations of the OH group directly attached to the heterocyclic ring. Such an observation provides a convincing evidence of a long-range vibrational energy transfer from the initially excited OH group (directly attached to the ring) to the remote hydroxymethyl fragment which changes its orientation. Structural changes, occurring in matrix-isolated molecules of kojic acid upon near-IR excitation, were monitored by FTIR spectroscopy.

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