Missing conformers. Comparative study of conformational cooling in cyanoacetic acid and methyl cyanoacetate isolated in low temperature inert gas matrixes

Reva, Igor; Stepanian, S. G.; Adamowicz, Ludwik; Fausto, Rui

doi:10.1016/s0009-2614(03)00782-6

Cited by 105 publications

(136 citation statements)

References 18 publications

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“…It also depends on the matrix material and increases for the series Ar, Kr, Xe. 22 In addition to spontaneous interconversion, transitions between conformers in matrix are known to be induced by infrared radiation, 19,23 such as from a Fourier transform infrared spectrometer. This is of much less concern in He droplets due to short time of flight during which the embedded molecules are only exposed to a single laser pulse.…”

Section: ͑4͒mentioning

confidence: 99%

Using He droplets for measurements of interconversion enthalpy of conformers in 2-chloroethanol

Skvortsov

Vilesov

2009

The Journal of Chemical Physics

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2-chloroethanol molecules having temperature from 300 to 600 K have been captured by helium droplets and the infrared spectra of the O–H and C–H stretching bands of the embedded molecules have been obtained. The intensity ratio of the bands due to trans and gauche conformers of the molecules follows the Arrhenius dependence, giving the enthalpy of conformers interconversion of 1.1 kcal/mol. It is concluded that the abundance ratio of the conformers, which are at equilibrium in the gas phase, remains unchanged upon the rapid cooling of the molecules in the helium droplets.

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Section: ͑4͒mentioning

confidence: 99%

Using He droplets for measurements of interconversion enthalpy of conformers in 2-chloroethanol

Skvortsov

Vilesov

2009

The Journal of Chemical Physics

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“…However, if thermally activated, this relaxation may occur over the barrier. 15 Indeed, the TtfCc transformation was induced by annealing of the sample to a temperature of 28 K! The rearrangement of the argon atoms in this thermal back conversion process should be nonsignificant, since the changes of the baseline in the experimental spectra observed upon annealing were negligible.…”

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confidence: 99%

Conformational Switching Induced by Near-Infrared Laser Irradiation

2009

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Hydroxyacetone (HA) is an important starting material in the organic synthesis of gem-diols, acetals, and ketals. 1 If such molecules are synthesized in interstellar medium, they may lead to the production of complex prebiotic chiral molecules, such as hydroxy aldehydes (sugars) and hydroxy acids. 2 HA is formed in the atmosphere during oxidation of the important biogenic hydrocarbon, isoprene. 3 Atmospheric HA reacts with an OH radical under sunlight irradiation 4 and also contributes to the formation of secondary organic aerosols. 5 The overall rate coefficient for the tropospherically relevant reaction, HA+OH, was proposed 6 to depend on the initial conformation of the HA molecule.Theoretical calculations on HA monomers in the gas phase 7,8 show that there are only two conformers (Cc and Tt) of this compound ( Figure 1). The lowest energy conformer of HA is Cc, whereas Tt was predicted to be higher in energy by 994 cm -1 . Hence, the Cc conformer should represent 99% of the conformational equilibrium at room temperature, 8 which is why only the Cc conformer has been observed so far. 2,8,9 In this work, we report the first experimental observation of the elusive high-energy Tt conformer of hydroxyacetone. We show that, for HA isolated in a low-temperature Ar matrix, the Tt conformer can be produced by narrow-band near-infrared (NIR) vibrational excitation of the conformational ground state Cc. When isolated in an argon matrix at 15 K, the photogenerated Tt form is stable. In the present study, the NIR-induced back transformation of the Tt conformer into the Cc form was also observed. This photoreversibility makes the HA molecule a system satisfying the criterion of a molecular switch. 10 In the experiments, tunable NIR radiation was provided by a Quanta-Ray MOPO-SL optical parametric oscillator (fwhm ∼0.2 cm -1 , repetition rate 10 Hz, pulse energy ∼3 mJ) pumped with a pulsed Nd:YAG laser. HA (Aldrich, >90%) was premixed with argon in a 1:1000 ratio and deposited onto a CsI window at 15 K. The FTIR spectra were recorded with 0.5 cm -1 resolution, in the 4000-400 cm -1 range. Other details of the experimental setup and of calculations are described elsewhere. 8 In a freshly deposited argon matrix the HA monomer only exists in Cc geometry, which is separated by a global barrier of 1417 cm -1 from the higher-energy Tt form. 8 Narrow-band NIR irradiation of the matrix at 6785 cm -1 led to a decrease of bands in the IR spectrum of the Cc form. Simultaneously, a set of new bands appeared due to a photogenerated species. The observed changes in the IR spectrum of HA are shown in Figures 1a and 2a.Comparison of the experimental difference spectrum with those calculated for the two conformers of HA (Figures 1b and 2b) reveals doubtlessly that the photogenerated species corresponds to the higher energy Tt conformer. The assignment of the nine most intense infrared Tt absorptions (along with Cc) is shown in Table 1.The observed shifts of the νOH and νCdO modes to higher frequencies (hypsochromic shifts) result from disru...

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“…Obviously, water impurities have longer relaxation times and longer diffusion paths on the Kr surface than with Ar. It is interesting to note in this connection the result of Reva et al [27] who demonstrated depletion of the low-barrier (~0.7 kcal/mole) cyanoacetic acid gauche conformers in the sequence of matrices Xe, Kr, Ar. It can be therefore inferred that relaxation on the surface of matrices plays an important role in depopulation of the low-barrier conformers.…”

Section: Resultsmentioning

confidence: 80%

“…One can see that, by extrapolations the reported experimental data [7,27], the conformers with barriers of about 0.7 kcal/mole can be easily conserved in the isolated state at 6 K. But it is possible only if the energy relaxation is very fast, otherwise, the population of conformers will change. Among the above-mentioned methods (HENDI, CRDS, and matrix isolation), the process of cooling is the longest in CRDS, which used seeded molecular beams, since gas-dynamic cooling occurs due to numerous collisions in the supersonic part of nozzle [29].…”

Section: Ftir Spectra and Conformations Of 2¢-deoxyuridine In Kr Matrmentioning

confidence: 81%

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FTIR spectra and conformations of 2′-deoxyuridine in Kr matrices

Ivanov

Karachevtsev

2007

Low Temperature Physics

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The Fourier transform infrared spectra in the range 4000-200 ñì -1 of pyrimidine nucleoside 2¢-deoxyuridine (dU) have been obtained in the low temperature inert Kr matrices. For the first time, instead of a usual flat mirror, a low temperature one-coordinate retroreflector was used as the matrix substrate. Owing to this, the matrix setup is insensitive to dip angle vibrations of the cryostat and is favourable to work with thinner matrix layers. Of two syn-conformers with dU_s1 and dU_s2 (stabilized by the intramolecular hydrogen bond O5¢H…O2), only dT_s2 conformer with Ñ2¢-endo structure of the ribose ring was uniquely quenched. The height of the interconversion barrier of the minor syn-conformer dU_s1 was estimated to be below 0.7 kcal/mole. It was shown that the energy relaxation of impurities in Kr is slower than in Ar matrices.

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Missing conformers. Comparative study of conformational cooling in cyanoacetic acid and methyl cyanoacetate isolated in low temperature inert gas matrixes

Cited by 105 publications

References 18 publications

Using He droplets for measurements of interconversion enthalpy of conformers in 2-chloroethanol

Using He droplets for measurements of interconversion enthalpy of conformers in 2-chloroethanol

Conformational Switching Induced by Near-Infrared Laser Irradiation

FTIR spectra and conformations of 2′-deoxyuridine in Kr matrices

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