Conformations of trimethyl phosphite: a matrix isolation infrared and ab initio study

The conformations of trimethyl phosphite (TMPhite) were studied using matrix isolation infrared spectroscopy. TMPhite was trapped in a nitrogen matrix using an effusive source maintained at two different temperatures (298 and 410 K) and a supersonic jet source. The experimental studies were supported by ab initio computations performed at the B3LYP/6-31++G** level. Computations identified four minima for TMPhite, corresponding to conformers with C(1)(TG(±)G(±)), C(s)(TG(+)G(-)), C(1)(G(±)TT), and C(3)(G(±)G(±)G(±)) structures, given in order of increasing energy. Computations of the transition state structures connecting the C(s)(TG(+)G(-)) and C(1)(G(±)TT) conformers to the global minimum C(1)(TG(±)G(±)) structure were also carried out. The barriers for the interconversion of C(s)(TG(+)G(-)) and C(1)(G(±)TT) to the ground state C(1)(TG(±)G(±)) conformer were 0.2 and 0.6 kcal/mol, respectively. Comparison of conformational preferences of TMPhite with the related carbon compound, trimethoxymethane, and the organic phosphate, trimethyl phosphate, was also made using natural bond orbital analysis.     

Conformations of trimethoxymethylsilane: matrix isolation infrared and ab initio studies

Conformations of trimethoxymethylsilane were studied using matrix isolation infrared spectroscopy and ab initio computations. Trimethoxymethylsilane was trapped in both argon and nitrogen matrixes using heated nozzle effusive sources and a supersonic jet source, in an effort to alter the conformational population in the matrix. Ab initio calculations were carried out at the HF and B3LYP level using 6-31++G basis set to support our experimental observations. The frequencies computed at the B3LYP level was found to fit well with our experimental data. A conformer with a C1(g(+/-)g(+/-)t) structure was predicted by our computations to be the ground state conformer.     

Conformations of dimethoxydimethylsilane: matrix isolation infrared and ab initio studies

Conformations of dimethoxydimethylsilane (DMDMS) were studied using matrix isolation infrared spectroscopy, by trapping the silane in argon and nitrogen matrixes. The matrix was deposited using both an effusive and a supersonic jet source. The effusive source was maintained at two different temperatures, viz. 298 and 433 K, during deposition to alter the conformational population of the silane. The experimental results were supported by computations performed at both the HF and B3LYP levels, using 6-31++G** basis set. Vibrational frequency calculations were carried out to assign the experimental features and also to ensure that the computed structures did indeed correspond to minima. A conformer with a G+/-G-/+ structure was found to be the ground state, while G+/-T and G+/-G+/- structures were the next higher energy conformers with energies of 1.32 and 1.48 kcal/mol, respectively. Natural bond orbital analysis was carried out at both HF/6-31++G** and B3LYP/6-31++G** level which indicated that the charge-transfer hyperconjugative interactions largely determine the conformational preferences in this molecule. This interaction appears to be smaller in DMDMS than in the corresponding carbon analogue, dimethoxypropane (DMP).     

Conformations of dimethoxymethane: matrix isolation infrared and ab initio studies

Conformations of dimethoxymethane (DMM) were studied using matrix isolation infrared spectroscopy. DMM was trapped in an argon matrix using an effusive source at 298, 388 and 430 K. Experiments were also done using a supersonic jet source to look for conformational cooling in the expansion process. As a result of these experiments, spectrally resolved infrared features of the ground and first higher energy conformer of DMM have been recorded, for the first time. The experimental studies were supported by ab initio computations performed at HF and B3LYP levels, using a 6-31++G** basis set. Computationally, four minima were identified corresponding to conformers with GG, TG, G+G- and TT structures. The computed frequencies at the B3LYP level were found to compare well with the experimental matrix isolation frequencies, leading to a definitive assignment of the infrared features of DMM, for the GG and TG conformers. At the B3LYP/6-31++G** level, the energy difference between the GG and TG conformers was computed to be 2.30 kcal mol(-1). The barrier for conformation interconversion, TG-->GG level was calculated to be 0.95 kcal mol(-1). This value is consistent with the experimental observation that the spectral features due to the TG conformer disappeared in the matrix on annealing.     

Conformations of dimethylhydrogen phosphonate (DMHP): a matrix isolation infrared and ab initio study

The infrared spectra of dimethylhydrogen phosphonate (DMHP) isolated in nitrogen, argon and krypton matrices using an effusive source at 298 and 373 K have been recorded. Experiments were also performed using a supersonic jet source to look for conformational cooling in the expansion process. As a result of these experiments, infrared spectral characteristics of the ground and higher energy conformers of the DMHP have been identified for the first time. The structures of DMHP were optimized at the hybrid B3LYP and Hartree fock (HF) levels of theory using the 6-31++G** basis sets. Computationally, four minima were obtained corresponding to DMHP conformers with G (+/-)G (-/+), G (-)G (-), TG (+) and TG (-) structures in the order of increasing energy. Frequency calculations were done to confirm that the structures were indeed minima on the potential energy surface (PES). The computed frequencies corroborated well with the experimental matrix isolation infrared frequencies leading to definite assignments of the infrared features of DMHP, for the G (+/-)G (-/+) and TG (+) conformers. At B3LYP/6-31++G** level, the energy difference between the G (+/-)G (-/+) and G (-)G (-) conformer was 1.53 kcal/mol, and that between G (+/-)G (-/+) and TG (+), G (+/-)G (-/+) and TG (-) were 1.65 and 1.95 kcal/mol. Transition-state calculations were also carried out at B3LYP/6-31++G** level connecting the G (+/-)G (-/+) to G (-)G (-), TG (+) and TG (-) conformers. Computations indicated that the conformer interconversion between G (-)G (-) --> G (+/-)G (-/+) is barrierless, whereas the barriers for TG (+) --> G (+/-)G (-/+) and TG (-) --> G (+/-)G (-/+) are 1.47 and 0.88 kcal/mol, respectively.     

Furan-formic acid dimers: an ab initio and matrix isolation study

The dimers formed by formic acid (FA) and furan are investigated by ab initio methods and matrix isolation spectroscopy. Nine complexes with binding energies between -3.91 and -0.82 kcal/mol (MP2/6-311++G(d,p) + ZPE + BSSE) are identified. Another five weaker bound complexes are localized at lower level of theory only. The binding in the furan-FA dimers can be described in terms of OH...O, C=O...H, HO...H, CH...O, OH...pi, and CH...pi interactions. Therefore, the furan-FA complexes are classified in two types: (1) the dimers where the OH hydrogen of formic acid interacts with the furan molecule and (2) the dimers where the main interactions of FA with the furan molecule are via the less acidic CH hydrogen. Duning's and Pople's triple and double basis sets were used to study the dependence of the geometries and energies of the complexes from the basis set. BSSE (basis set superposition error) counterpoise corrections (CP) were included during the geometry optimizations of all dimers at the MP2/6-31G(d,p) level of theory. Matrix isolation spectroscopy allowed us to record the IR spectrum of aggregates between FA and furan. By comparison of the experimental IR spectrum with calculated IR spectra of a variety of complexes, it was possible to identify the most stable furan-FA dimer as the major product of the aggregation.     

A combined matrix isolation and ab initio study of bromine oxides

Bromine oxides have been generated by passing a mixture of Br(2)/O(2)/Ar through a microwave discharge. The products were stabilized at 6.5 K in an excess amount of argon. Infrared spectroscopy was used to analyze the species formed; experiments with enriched (18)O(2) and ab initio calculations were carried out to assist in the assignment of the spectra. Besides the known species BrO, OBrO, and BrBrO, spectroscopic evidence for BrOBrO, BrBrO(2), and a new isomer of Br(2)O(3) is reported for the first time. Extensive comparisons are drawn between the present studies and previous experimental and theoretical works. The chemistry involved in the production of the observed compounds is discussed. The assignments are corroborated by the good correlation between observed and calculated band positions and intensities.     

The structures of some trimers of carbon monoxide--an infrared matrix isolation spectroscopic and ab initio molecular orbital study

Using a number of potential models for the gas-phase structure of the trimer of carbon monoxide as a guide, ab initio molecular orbital calculations have been carried out on this aggregate in order to determine its probable structure and vibrational spectrum in cryogenic matrices. The Fourier transform infrared spectra of carbon monoxide trapped in argon and nitrogen matrices have been recorded and, on the basis of the results of the theoretical calculations, a search for possible absorptions which may be assigned to trimeric species has been undertaken.     

Matrix isolation infrared and ab initio study of the conformations of 2,2-dimethoxypropane

Conformations of 2,2-dimethoxypropane (DMP) were studied using matrix isolation infrared spectroscopy. An effusive source maintained at different temperatures (298, 388 and 430 K) was used to deposit DMP in a nitrogen matrix. As a result of these experiments, spectrally resolved infrared features of the ground and first higher energy conformer of DMP have been recorded, for the first time. The experimental studies were supported by ab initio computations performed at B3LYP/6-31++G** level. Computationally, four minima were identified corresponding to conformers with G+/-G-/+, TG+/-, G+/-G+/- and TT structures. The computed frequencies at the B3LYP level were found to compare well with the experimental matrix isolation frequencies, leading to a definitive assignment of the infrared features of DMP, for the G+/-G-/+ and TG+/- conformers. At the B3LYP/6-31++G** level, the energy difference between the G+/-G-/+ and TG+/- conformers was computed to be 3.25 kcal x mol(-1). The barrier for conformation interconversion, TG+/--->G+/-G-/+, was calculated to be 1.29 kcal x mol(-1). The magnitude of this barrier is consistent with the experimental observation that the spectral features due to the TG+/- decreased considerably in intensity when the matrix was annealed.     

Tautomerism and infrared spectra of 2-thiopurine: an experimental matrix isolation and theoretical ab initio and density functional theory study

Infrared spectra of 2-thiopurine (2-mercaptopurine, 2-purinethiol ) isolated in low-temperature Ar and N2 matrixes are reported. These spectra indicate that the compound adopts exclusively the thiol N9H tautomeric form. The theoretical calculations of relative energies of 2-thiopurine tautomers have been carried out at the MP4(SDTQ)//HF level using the 6-31G(d,p) basis set. The thiol N9H tautomer was predicted to be the most stable of all isomers of 2-thiopurine. The infrared spectra of the tautomers of 2-thiopurine have been calculated at the DFT(B3LYP)/6-31G(d,p) level. Good agreement between the experimental spectra and the spectra calculated for thiol N9H tautomer supported the identification of the dominant tautomer. It has also allowed for the reliable assignment of the bands observed in the experimental IR spectrum.     

Vibrational frequencies and infrared intensities of acetonitrile coordinated with metal cations: an ab initio study

Vibrational frequencies and infrared intensities have been calculated at the 6-31G and 6-31G^** levels for acetonitrile and for the complexes of acetonitrile with Li⁺ and Na⁺ cations. The changes in the infrared characteristics from an isolated acetonitrile to acetonitrile coordinated with metal cations (Li⁺ and Na⁺) have been evaluated. The ab initio calculations predict an essential increase of the intensities of the stretching CN, C-C and deformation CH₃, CCN vibrations in the complexes of acetonitrile with Li⁺ and Na⁺ cations.     

Matrix isolation infrared and ab initio studies on conformers of 3-fluoropropene

The infrared spectra of the cis and gauche conformers of 3-fluoropropene, CH₂CHCH₂F, were studied in Ne, Ar, Kr and Xe matrices. An infrared-induced cis to gauche rotamerization was found in Ar, Kr and Xe matrices. A thermal interconversion process was also found. Its direction was dependent upon the host, being the same as that of the IR process in Kr but reverse in Ar and Xe. In Ar and Xe matrices considerable site-splitting occurs in the IR spectra and a detailed analysis of the processes in different sites is given. An energy difference of 2.5±0.3 kJ mol⁻¹ between the cis and gauche species was obtained on assuming that the gas phase equilibrium between the conformers is trapped upon deposition. A slow dark process from cis to gauche conformer was observed in Kr matrices at temperatures above 15 K, possibly due to tunnelling. Ab initio calculations were carried out on 3-fluoropropene. The torsional potential energy curve and spectra of the conformers were calculated at the MP2(full)16-31G** level and were compared with the experimental results.     

Insertion of noble gas atoms into cyanoacetylene: an ab initio and matrix isolation study

A computational and experimental matrix isolation study of insertion of noble gas atoms into cyanoacetylene (HCCCN) is presented. Twelve novel noble gas insertion compounds are found to be kinetically stable at the MP2 level of theory, including four molecules with argon. The first group of the computationally studied molecules belongs to noble gas hydrides (HNgCCCN and HNgCCNC), and we found their stability for Ng = Ar, Kr, and Xe. The HNgCCCN compounds with Kr and Xe have similar stability to that of previously reported HKrCN and HXeCN. The HArCCCN molecule seems to have a weaker H-Ar bond than in the previously identified HArF molecule. The HNgCCNC molecules are less stable than the HNgCCCN isomers for all noble gas atoms. The second group of the computational insertion compounds, HCCNgCN and HCCNgNC, are of a different type, and they also are kinetically stable for Ng = Ar, Kr, and Xe. Our photolysis and annealing experiments with low-temperature cyanoacetylene/Ng (Ng = Ar, Kr, and Xe) matrixes evidence the formation of two noble gas hydrides for Ng = Kr and Xe, with the strongest IR absorption bands at 1492.1 and 1624.5 cm(-1), and two additional absorption modes for each species are found. The computational spectra of HKrCCCN and HXeCCCN fit most closely the experimental data, which is the basis for our assignment. The obtained species absorb at quite similar frequencies as the known HKrCN and HXeCN molecules, which is in agreement with the theoretical predictions. No strong candidates for an Ar compound are observed in the IR absorption spectra. As an important side product of this work, the data obtained in long-term decay of KrHKr+ cations suggest a tentative assignment for the CCCN radical.     

The combined use of ab initio molecular orbital theory and matrix isolation infrared spectroscopy in the study of molecular interactions

The infrared spectra of a linear (C_s) and a cyclic (C_i) water dimer and a linear (C_s) and a cyclic (C_2h) ammonia dimer have been calculated, at the second order level of MØller-Plesset perturbation theory, using the 6-31G^** basis set. The calculated spectra have been compared with the spectra of water and ammonia isolated in nitrogen and argon matrices. In the case of water, theory predicts the linear to be the more stable isomer, and the observed spectrum can only be interpreted in terms of the linear structure. For ammonia, while the experimentally determined spectrum fits the calculated spectrum of the linear dimer fairly closely, the prediction of which is the more stable structure is still ambiguous.     

Infrared spectra and tautomerism of isocytosine; an ab initio and matrix isolation study

Prototropic tautomerism of isocytosine has been investigated using both theoretical ab initio and experimental matrix isolation IR methods. The coexistence of the amino-hydroxy and amino-oxo N(3)H forms, with a clear predominance of the hydroxy form, was observed. The tautomerization constant [oxo]/[hydroxy] obtained from experimental and calculated IR intensities was 0.11 at the micro-oven temperature of 400 K. The ab initio prediction of the relative energies of the tautomers is in reasonable agreement with the experimental estimation. The change of the tautomeric form oxo→hydroxy upon UV irradiation was used to separate the IR spectra of both tautomers. A theoretically assisted interpretation of the IR spectra of both observed tautomers is proposed.     

Matrix isolation and ab initio study of the HXeCCH...CO2 complex

The HXeCCH...CO2 complex is studied experimentally and computationally. The complex is prepared in a low-temperature xenon matrix using UV photolysis of propiolic acid (HCCCOOH) and thermal mobilization of H atoms at 45 K. Photolysis of propiolic acid leads to the HCCH...CO2 complex as one of the photolysis products. The HCCH...CO2 complex is further photolyzed to the HCC...CO2 complex. Thermal annealing leads to the formation of HXeCCH complexed with CO2. The H-Xe stretching absorption of the HXeCCH...CO2 complex is blueshifted (+31.9 and +5.8 cm(-1)) from the value of the HXeCCH monomer in a xenon matrix. In the calculations, three HXeCCH...CO2 structures were found (one parallel and two linear structures) corresponding to the true energy minima on the potential energy surface. For the H-Xe stretching mode, the calculations give blueshifted values of +19.2 or +19.5 cm(-1) depending on the computational level [MP2/6-311++G(2d,2p) and MP2/aug-cc-pVDZ] for the parallel structure and +19.4 or +27.9 cm(-1) for one linear structure. For the second linear structure, the H-Xe stretching frequency is redshifted by -8.6 or -9.4 cm(-1) at these levels of theory. Based on the calculations, the experimental band shifted by +5.8 cm(-1) (1492.2 cm(-1)) most likely corresponds to the HXeCCH...CO2 parallel structure. The band with larger blueshift of +31.9 cm(-1) (1518.3 cm(-1)) can be due to another matrix site of the same structure or to the blueshifting linear structure.     

UV-laser photoisomerization of fumaryl chloride: the first identification of maleoyl chloride: matrix isolation infrared and ab initio studies

Fourier transform infrared spectra of fumaryl chloride 1 isolated in an argon matrix at 10 K have been analyzed. The comparison between the ab initio HF/6-31G calculated infrared spectra with the experimental ones reveals the existence of three planar conformers, the cis cis 1a, the cis trans 1b and the trans trans 1c. Laser UV irradiation of 1 at lambda = 340 nm yields maleoyl chloride 2 by a carbon carbon double bond photoisomerization process. The first identification of this compound was performed by comparison of the experimental infrared spectra with the calculated ones at the MP2/6-1G** level. AM1 semiempirical and ab initio calculations were used to calculate the structure and the relative stability of the three non planar maleoyl chloride conformers.     

The structures of some trimers of carbon monoxide—An infrared matrix isolation spectroscopic and ab initio molecular orbital study

Using a number of potential models for the gas-phase structure of the trimer of carbon monoxide as a guide, ab initio molecular orbital calculations have been carried out on this aggregate in order to determine its probable structure and vibrational spectrum in cryogenic matrices. The Fourier transform infrared spectra of carbon monoxide trapped in argon and nitrogen matrices have been recorded and, on the basis of the results of the theoretical calculations, a search for possible absorptions which may be assigned to trimeric species has been undertaken.     

A comparative ab initio study of ethylene,acetylene and benzene

Ground state properties have been calculated by use of a medium-sized Gaussian basis set and comparison with other bases has been made. Contraction to double-zeta of a comparatively small basis is found to be superior to a large set of primitive Gaussians contracted to minimal basis. Molecular optimization is not important for double-zeta bases. Inclusion of a balanced set of polarization functions is essential in all cases studied. Population analysis gives a certain insight in molecular properties but contour maps are found to be significantly superior. This is demonstrated on bonding properties of corresponding orbitals within the series. In case of benzene Slater's energyband plot is shown to be useful for classifying bonding properties.     

Vapor phase infrared spectroscopy and ab initio fundamental anharmonic frequencies of ammonia borane

The infrared absorption spectrum of ammonia borane vapor has been recorded between 3600 and 600 cm(-1). Of the eleven infrared active fundamental vibrational modes, seven modes of NH(3)(11)BH(3) and four modes of NH(3)(10)BH(3) were observed. The spectra were recorded with sufficient resolution to observe the rotational structure of the bands, which allowed for preliminary least-squares fitting of the band origins and rotational constants. First-principles electronic structure calculations were performed to obtain anharmonic band origins and their intensities. The band assignments are discussed in relation to other spectroscopic techniques that have been previously used to study this molecule. A semi-empirical estimate of the vapor pressure of ammonia borane at room temperature (22 °C) was made and found to be ~1 × 10(-4) Torr. The assignment of the measured modes was aided by the calculated anharmonic frequencies and their infrared intensities. The combination of the CCSD(T) harmonic frequencies with the B3LYP anharmonicities, obtained from second-order vibrational perturbation theory, was found to produce an overall best agreement with the measured band origins.