K.S. Viswanathan

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.

Matrix isolation-supersonic jet infrared spectroscopy: conformational cooling in trimethyl phosphate

A supersonic jet source has been coupled to a matrix isolation set-up to obtain conformational cooling in organic phosphates. Infrared spectra of trimethyl phosphate (TMP) trapped in a nitrogen matrix using supersonic jet sampling, indicated that the higher-energy conformer of TMP was significantly depopulated, compared with the spectra recorded using the conventional effusive source. The conformational temperature was estimated to be 147 K. This study demonstrates for the first time the successful use of supersonic jet sampling to obtain conformational cooling in molecules where the barrier to conformation interconversion is reasonably large (> ca. 10 kJ mol−1).

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.

An ab initio and matrix isolation infrared study of the 1:1 C2H2–CHCl3 adduct

Abstract The details of weak C–H⋯π interactions that control several inter and intramolecular structures have been studied experimentally and theoretically for the 1:1 C2H2–CHCl3 adduct. The adduct was generated by depositing acetylene and chloroform in an argon matrix and a 1:1 complex of these species was identified using infrared spectroscopy. Formation of the adduct was evidenced by shifts in the vibrational frequencies compared to C2H2 and CHCl3 species. The molecular structure, vibrational frequencies and stabilization energies of the complex were predicted at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels. Both the computational and experimental data indicate that the C2H2–CHCl3 complex has a weak hydrogen bond involving a C–H⋯π interaction, where the C2H2 acts as a proton acceptor and the CHCl3 as the proton donor. In addition, there also appears to be a secondary interaction between one of the chlorine atoms of CHCl3 and a hydrogen in C2H2. The combination of the C–H⋯π interaction and the secondary Cl⋯H interaction determines the structure and the energetics of the C2H2–CHCl3 complex. In addition to the vibrational assignments for the C2H2–CHCl3 complex we have also observed and assigned features owing to the proton accepting C2H2 submolecule in the acetylene dimer.

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.

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.

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.

Effect of ligand structure on synergism in Tb3+-aromatic acid complexes: fluorescence lifetime studies

The fluorescence of Tb3+ sensitized by aromatic carboxylic acid ligands (benzoic, monomethylphthalic, monomethylterephthalic, trimesic, terephthalic, isophthalic, phthalic and mellitic acids) and the synergism displayed by these complexes when treated with TOPO/Triton X-100 have been studied by measuring lifetimes of Tb3+ emission. The lifetime of Tb3+ fluorescence was not significantly altered following complex formation with aromatic carboxylic acids, even though a significant enhancement in the Tb3+ fluorescence intensity was observed in every single case. However, when these Tb3+-aromatic acid complexes were treated with TOPO/Triton X-100, the lifetimes of the Tb3+ fluorescence increased markedly, but only with certain acids. Interestingly, even amongst the acids that showed an increase in lifetime with TOPO/Triton X-100, the lifetimes as a function of the pH of the solution was strongly dependent on the structure of the ligand. These differences and the reasons for such behavior are discussed, which shed light on the role of the structure of the ligand on the synergism process.

Trimethyl phosphate-CCl4 interaction: experimental and computational evidence for Cl...O interactions

Abstract The formation of a weak Cl⋯O bond has been discerned both experimentally and computationally in our studies on the trimethyl phosphate (TMP)–CCl 4 system. TMP and CCl 4 were codeposited in a nitrogen matrix and adducts of these species were identified using infrared spectroscopy. Formation of the weak adducts were evidenced by shifts in the vibrational frequencies of TMP. The structures and the vibrational frequencies of these adducts were computed at the HF/6-31G ∗∗ level and the computed vibrational frequencies compared well with the observed frequencies. The nature of the bonding in this adduct was also studied using the atom-in-molecule theory, which clearly showed a (3, −1) bond critical point between the two submolecules. The stabilization energy of these adducts, were computed both at the HF/6-31G ∗∗ //HF/6-31G ∗∗ and MP2/6-31G ∗∗ //HF/6-31G ∗∗ levels after correcting for zero-point energies and basis set superposition errors.