K. Sundararajan

H⋯π complexes of acetylene–benzene: a matrix isolation and computational study

Abstract Hydrogen bonded H⋯π complexes of C 2 H 2 and C 6 H 6 were studied both computationally and experimentally. Computationally, C 2 H 2 –C 6 H 6 complexes of 1:1 and 2:1 stoichiometries were identified. The molecular structure and stabilisation energies of the complexes were calculated at the HF, MP2, MP2(Full) and B3LYP levels of theory employing basis sets ranging from 6-31G(d,p) and 6-31++G(d,p) while the frequency calculations were performed at HF, B3LYP and MP2 levels using 6-31G(d,p) and 6-31G++(d,p) basis sets. Using matrix isolation infrared spectroscopy, we observed a 1:1 adduct in an argon matrix. Formation of the adduct was evidenced by shifts in the vibrational frequencies of the acetylene and benzene submolecules in the complex. Though our computations showed two types of 1:1 complexes—one where the acetylene is the proton donor and another where the benzene is the proton donor, experimentally, we observed only the complex, where acetylene acts as a proton donor to the π cloud of benzene.

Effect of matrix on IR frequencies of acetylene and acetylene-methanol complex: Infrared matrix isolation and ab initio study

Effect of nitrogen and argon matrices on the C-H asymmetric stretching and bending infrared frequencies of the acetylene molecule, C(2)H(2), has been studied by matrix isolation experiments as well as by calculations at MP2 level of theory. The complexes of C(2)H(2) in nitrogen and argon matrices, viz., C(2)H(2)(N(2))(m) (with m=2-8) and C(2)H(2)(Ar)(n) (with n=2-10) are theoretically explored. The computed acetylenic C-H asymmetric stretch in C(2)H(2)-nitrogen complexes shows a redshift of 3.0 to 11.9 cm(-1) compared with the frequencies of the free acetylene molecule, and a corresponding blueshift of 7.4 to 26.2 cm(-1) when C(2)H(2) is complexed with argon atoms. The trends in the computed shifts are in good agreement with the experiments. The molecular electrostatic potential minimum of C(2)H(2) becomes more negative when complexed with nitrogen than on complexation with argon. This observation implies a greater basic character for C(2)H(2) in the nitrogen matrix, favoring the formation of H-pi(C(2)H(2)-MeOH) complex as compared to that in the Ar matrix. Experimentally the preferential formation of H-pi(C(2)H(2)-MeOH) complex in the N(2) matrix has indeed been observed.

Conformations of dimethoxymethane: matrix isolation infrared and ab initio studies

Abstract 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.

Experimental evidence for blue-shifted hydrogen bonding in the fluoroform-hydrogen chloride complex: a matrix-isolation infrared and ab initio study.

The 1:1 hydrogen-bonded complex of fluoroform and hydrogen chloride was studied using matrix-isolation infrared spectroscopy and ab initio computations. Using B3LYP and MP2 levels of theory with 6-311++G(d,p) and aug-cc-pVDZ basis sets, the structures of the complexes and their energies were computed. For the 1:1 CHF3-HCl complexes, ab initio computations showed two minima, one cyclic and the other acyclic. The cyclic complex was found to have C-H · · · Cl and C-F · · · H interactions, where CHF3 and HCl sub-molecules act as proton donor and proton acceptor, respectively. The second minimum corresponded to an acyclic complex stabilized only by the C-F · · · H interaction, in which CHF3 is the proton acceptor. Experimentally, we could trap the 1:1 CHF3-HCl cyclic complex in an argon matrix, where a blue-shift in the C-H stretching mode of the CHF3 sub-molecule was observed. To understand the nature of the interactions, Atoms in Molecules and Natural Bond Orbital analyses were carried out to unravel the reasons for blue-shifting of the C-H stretching frequency in these complexes.

Matrix isolation infrared and DFT study of the trimethyl phosphite-hydrogen chloride interaction: hydrogen bonding versus nucleophilic substitution.

Trimethyl phosphite (TMPhite) and hydrogen chloride (HCl), when separately codeposited in a N(2) matrix, yielded a hydrogen bonded adduct, which was evidenced by shifts in the vibrational frequencies of the TMPhite and HCl submolecules. The structure and energy of the adducts were computed at the B3LYP level using 6-31++G** and aug-cc-pVDZ basis sets. While our computations indicated four minima for the TMPhite-HCl adducts, only one adduct was experimentally identified in the matrix at low temperatures, which interestingly was not the structure corresponding to the global minimum, but was the structure corresponding to the first higher energy local minimum. The Onsager self-consistent reaction field model was used to explain this observation. In an attempt to prepare the hydrogen bonded adduct in the gas phase and then trap it in the matrix, TMPhite and HCl were premixed prior to deposition. However, in these experiments, no hydrogen bonded adduct was observed; on the contrary, TMPhite reacted with HCl to yield CH(3)Cl, following a nucleophilic substitution, a reaction that is apparently frustrated in the matrix.

A general parallel algorithm for the generation of moleular electrostatic potential maps

Abstract Application of rigorous bounds yields an algorithm approximately linear in the number of primitive Gaussians for the computation of the molecular electrostatic potential (MESP). Careful strategies for parallelization of the algorithm have been evolved. Linear as well as superlinear speedups have been achieved by effective load balancing, viz. even distribution of the workload amongst the worker processors. The parallel algorithm was tested on C 2 H 4 , C 3 H 4 , C 3 H 6 , C 4 H 6 , C 6 H 6 , C 6 H 5 NO 2 , and C 10 H 8 molecules using 4–31G ** , 4–31G and 3G basis sets. The generation of the MESP on 100 × 100 points, for any of these molecules, using the parallel algorithm requires less than 5 min on the 32-node INMOS T800-based Parsytec computing system.

Conformations of triethyl phosphate: a supersonic jet-matrix isolation and semi-empirical (AM1) study

Abstract A supersonic jet source coupled to a matrix isolation set-up was used to obtain conformational cooling in triethyl phosphate (TEP). Infrared spectra of the supersonically expanded TEP trapped in a nitrogen matrix revealed depopulation of the higher energy conformation in the beam. As a result of this work, vibrational features corresponding to the ground state and higher energy conformations of TEP were identified. AM1 computations indicated two distinct and nearly degenerate structures with C1 symmetries for the ground state conformation of TEP. The assignment of a low symmetry for the ground state structures was supported by vibrational spectra obtained using our supersonic jet-matrix isolation set-up. Possible structures were also proposed for the higher energy conformers.

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.

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.

Trimethyl phosphate-acetylene interaction: a matrix-isolation infrared and ab initio study.

Trimethyl phosphate (TMP) and acetylene were codeposited in nitrogen and argon matrices and adducts of these species were identified using infrared spectroscopy. Formation of the adducts was evidenced by shifts in the vibrational frequencies of the modes involving the TMP and acetylene submolecules. The structures of these adducts, energies and the vibrational frequencies were computed at the HF/6-31G** level. Both the experimental and computational studies indicated that two types of TMP-acetylene complexes were formed; one in which the hydrogen in acetylene was bonded to the phosphoryl oxygen and another in which the bonding was at the alkoxy oxygen of the phosphate. In addition to the primary hydrogen bonded interaction at the phosphoryl oxygen, this complex, also appeared to be stablilized by a secondary and weaker interaction involving a methyl hydrogen in TMP and the pi cloud in acetylene--a case of a H...pi interaction. The computed vibrational frequencies in the adducts agreed well with the observed frequencies for the modes involving the TMP submolecule, while the agreement was relatively poor for the modes involving the acetylene submolecule. The stabilization energies of these adducts, corrected for both zero-point energies and basis set superposition errors, were approximately 3 kcal/mol for the phosphoryl complex and, approximately 1 kcal/mol for the alkoxy complex.