Low-temperature Ar Matrices Research Articles

Electronic and vibrational spectroscopy of phototautomerizing heteroazaaromatics with intramolecular hydrogen bonds

Four phototautomerizing derivatives of bipyridyl with intramolecular single and double hydrogen bonds are studied by IR and electronic spectroscopy. A broad IR band corresponding to the stretching vibration of the OH group engaged in the N⋯HO hydrogen bond is observed in all compounds in solutions, KBr tablets and low-temperature Ar matrices. Variations of excited state behaviour observed among the four molecules are well correlated with the ground state hydrogen bonding characteristics.

A spectroscopic study of the reaction of the carbene, tetrabromocyclopentadienylidene, with O2 in low temperature matrices

Abstract The photolysis of tetrabromodiazocyclopentadiene (2) in low temperature Ar and N2 matrices generates the carbene, tetrabromocyclopentadienylidene (3). In O2-doped matrices, (3) reacts with oxygen yielding first tetrabromocyclopentadienone O-oxide (4), identification of which was aided by experiments with isotopically labelled oxygen, and then tetrabromocyclopentadienone (5) and tetrabromo-α-pyrone (6). The matrix uv-visible and infrared absorption spectra of tetrabromocyclopentadienone O-oxide (4) are reported.

Matrix isolation infrared and ab initio studies on conformers of formic acid anhydride

High-energy conformers of formic acid anhydride were produced in low-temperature Ar matrices with the aid of UV irradiation. One previously unknown conformer, EE, was detected from the FTIR spectra of the irradiated matrices. Connected with the assignment of this new species, the stabilities and vibrational spectra of three previously assumed conformers, obtained with respect to internal rotation about the C-O bonds, were studied by ab initio methods. At all levels used, the AE conformer was the lowest energy species

Observation of the Infrared and Raman Spectra of p-Benzoquinodimethane in Low-Temperature Ar Matrices

Abstract The infrared and Raman spectra of p-benzoquinodimethane have been observed in low-temperature Ar matrices. The infrared spectrum obtained has proved to be more reliable and useful than the existing numerical data on the absorption wavenumbers and qualitative intensities. The Raman spectrum has been observed for the first time. Vibrational assignments of bands characteristic of this quinoid molecule are discussed on an experimental basis.

ChemInform Abstract: Reinvestigation of IR‐Induced Conformational Isomerizations of 1,2‐ Ethanediol in Low‐Temperature Ar Matrices and Reverse Reaction in the Dark.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

FT-IR spectroscopic study of uracil derivatives and their hydrogen bonded complexes with proton donors: Part III. Hydrogen bonding of uracils with H 2O in argon matrices

The hydrogen bond interaction of uracils (uracil, 1-CH 3-uracil, 3-CH 3-uracil, 5-CH 3-uracil, 1,3-diCH 3-uracil) and thiouracils (2-thiouracil, 4-thiouracil, 1-CH 3-2-thiouracil, 1-CH 3-5-Br-2-thiouracil) with water is investigated using matrix-isolation FT-IR spectroscopy. It is demonstrated that H 2O forms both CO⋯HO and NH⋯OH complexes in low temperature Ar matrices. No experimental evidence is obtained for the existence of a closed CO⋯HO⋯HN heterodimer which has been predicted from ab initio calculations. Different amounts of C 2O and C 4O, and of N 1H and N 3H complexes are formed in the matrix depending on the substitution (methyl, thio, halogen) on the uracil ring. The monomer assignments in earlier reports are supported by the shifts observed on H-bonding with H 2O.

IR-induced conformer interconversion processes of glycolaldehyde in low-temperature matrices, and ab initio calculations on the energetics and vibrational frequencies of the conformers

Both IR-induced and thermal conformer interconversion processes of glycolaldehyde (CHOCH 2OH) were studied in low-temperature Ar, Kr and Xe matrices. The observed photochemical reaction induced by broad-band IR irradiation is interpreted to proceed from the lowest-energy conformer Cc which prevails after deposition to conformer Tt. In Xe and Kr the photochemical steady state (ratio [Tt]/[Cc]) depends on the matrix temperature; two conformers, Tg and Tt, seem to be in thermal equilibrium, and at elevated temperatures species Tg is enriched, and then the reverse photoprocess Tg → Cc occurs more rapidly than the competing reverse process, Tt → Cc which prevails at 13 K. Because of the thermal equilibrium the routes of the forward and reverse photoprocesses are considered to be different. A thermal reverse process, Tt → Tg → Cc, is observed at temperatures above 30 K in Xe and Kr. The role of the torsional barrier heights in determining the rates of the photoprocesses is emphasized. To aid in the interpretation of the experimental results, ab initio calculations were carried out. Geometries and energies of all conformers of glycoaldehyde as well as the geometries of the most important saddle points were fully optimized at the HF/6-31G** level, and vibrational spectra were calculated on the HF/4-31G level; the latter conformed to the experimental conformer assignment.

IR-induced photorotamerization of allyl alcohol in low-temperature matrices and ab initio calculations

The infrared regions νOH, τOH, and 950–900 cm−1 of allyl alcohol (2-propen-l-ol) were studied in low-temperature Ar, Kr, Xe, N2, and CO matrices. A fast IR-induced process and a similar slow dark process were found. It is concluded that the photoprocess is a rotamerization (obviously Cg→Gg′), since the process was similar in all five matrix media and also for other allyl compounds. The photoprocess was found to obey the first order rate law. Radiation from 9000 to below 2600 cm−1 was found to be active in initiating the process, especially the radiation corresponding to the νOH and methylenic νCH2 regions. The process is very fast in xenon. A possible mechanism for the process is discussed. To aid the interpretation of the results, ab initio calculations were carried out. All five conformers of allyl alcohol were optimized on the 4-31G level, and the corresponding semirigid potential energy curves of the torsional coordinates were constructed. Some calculations were carried out also on the 6-31G**, MP/4-31G, and CID/4-31G levels, using the 4-31G geometries. These calculations indicated that the conformers Gg′ and Cg are the lowest-energy species of quite similar energies.

Matrix-isolation infrared and ultraviolet spectroscopic studies of less stable conformers of 1,3,5-hexatriene

The infrared spectra of the 3- trans and 3- cis isomers of 1,3,5-hexatriene in low-temperature Ar matrices deposited from a high-temperature nozzle or deposited under irradiation of the Hg 253.7 nm light show a number of new bands. Correspondingly, the ultraviolet spectra of the 3- trans isomer observed under similar experimental conditions show a new absorption at 276 nm. Most of these new infrared bands and the new ultraviolet absorption are attributable to less stable isomers which have the planar s- cis conformation [or gauche conformation(s) close to the planar s- cis] about either one or both of the two CC bonds.

Infrared-induced conformational isomerization of ethylene glycol in a low-temperature argon matrix

The infrared spectra of ethylene glycol and its three deuterated species trapped in low-temperature Ar matrices show irreversible changes upon exposure to the radiation from the light source of an infrared spectrophotometer. Spectral changes in the O-H stretching region demonstrate the existence of two intramolecularly hydrogen-bonded conformers, namely, TGG′ and GGG′, which are predominant immediately after deposition but diminish under irradiation through isomerization to other conformers. Analysis of the spectral changes in the C-O stretching and OH bending regions suggests that the extended all-trans conformer, TTT, is the main product of isomerization. Therefore, the main pathways of conformational isomerization under study are those which go from both TGG′ and GGG′ to TTT. This conclusion is consistent with more detailed vibrational assignments based on normal-coordinate calculations. Further, the intensities of bands assigned to TGG′ are found to decrease faster than those of bands assigned to GGG′. Experiments with filter show that the infrared light between 3950 and 3600 cm−1, especially in the 3700-3600 cm−1 region, is responsible for the conformational isomerization. A possible isomerization mechanism is proposed.

Molecular Force Fields of s-trans-1,3-Butadiene and the Second Stable Conformer

Abstract Normal-coordinate analyses of s-trans-1,3-butadiene and its various deuterated and 13C-substituted analogs were performed using the group-coordinate force field, and a set of force constants was determined. The infrared spectra of the second stable conformer of 1,3-butadiene-d0, -1,1,4,4-d4, and -d6 were analyzed on the basis of normalcoordinate calculations of the planar s-cis form and a nonplanar gauche form. Such an analysis suggests that the second stable conformer takes a gauche form rather than the s-cis. To increase the number of reliable data to be used in this study, the infrared and Raman spectra of the d0, 1,1,4,4-d4, and d6 species were observed in low-temperature Ar matrices. The infrared spectra of the second stable conformer were also observed using a combination of the high-temperature nozzle and low-temperature matrix techniques.