Sundaram Arulmozhiraja

Torsional barrier, ionization potential, and electron affinity of biphenyl—A theoretical study

The structure and torsional barriers at 0° and 90° for biphenyl were studied by both ab initio and density functional methods by using various levels of theory with different basis sets. The torsional angle (φ) calculated at the MP2/6-311+G(2d,2p) level was 42.1°, while φ calculated using various density functionals with different basis sets was close to 40°. In contrast with the ab initio results, the torsional barrier at 0° [ΔE0=E(φ=0°)−E(equilibrium)] obtained using various density functionals coincided well with experimental values. The torsional barrier ratio (ΔE90/ΔE0) obtained at the B3LYP/cc-pVTZ level, 1.0988, agreed well with the experimental value, 1.0833, whereas it was 0.416 at the MP2/6-311G(d,p) level. Structural studies on biphenyl ions showed that the biphenyl cation has a nonplanar (φ=19°) structure whereas its anionic counterpart has a planar structure. The ionization potential obtained at the B3LYP/6-311+G(2d,2p) level was 7.86 eV. Contrary to an earlier study, a positive electron affi...

Density functional theory-based reactivity descriptors for dioxins

Reactivity descriptors based on density functional theory were used to analyse the toxicities and to predict the reactive sites of various dioxins at the B3LYP/6-311G(d,p) level. Many polychlorinated dibenzo-p-dioxins, including 2,3,7,8-tetrachloro dibenzo-p-dioxin (TCDD), 2,3,7,8-tetrafluoro dibenzo-p-dioxin (TFDD) and 2,3,7,8-tetrabromo dibenzo-p-dioxin (TBDD), were considered for this purpose. The present study showed that higher toxicity dioxins had a large atomic charge in their lateral part. The global softness values for the more toxic dioxins were larger than those of the less toxic dioxins and, thus, the former should be more polarizable than their less toxic counterparts. Local softness values revealed that chlorine atomic sites in the toxic dioxins had the highest electrophilicity and these sites may initiate the interaction with aromatic hydrocarbon (Ah) receptors. Interaction between dioxin and Ah receptors was studied by considering three amino acids. Reactivity descriptors for the three selected amino acids were obtained to determine the most probable sites of their interactions with the dioxins. The results revealed that dioxins were electron acceptors in their interactions with receptors. The 2,3,7,8-TFDD may be less toxic, while its brominated counterpart is at least as toxic as 2,3,7,8-TCDD.

Electronic spectra and photodissociation of vinyl chloride: A symmetry-adapted cluster configuration interaction study

The vertical absorption spectrum and photodissociation mechanism of vinyl chloride (VC) were studied by using symmetry-adapted cluster configuration interaction theory. The important vertical pi --> pi* excitation was intensively examined with various basis sets up to aug-cc-pVTZ augmented with appropriate Rydberg functions. The excitation energy for pi --> pi* transition obtained in the present study, 6.96 eV, agrees well with the experimental value, 6.7-6.9 eV. Calculated excitation energies along with the oscillator strengths clarify that the main excitation in VC is the pi --> pi* excitation. Contrary to the earlier theoretical reports, the results obtained here support that the C-Cl bond dissociation takes place through the n(Cl-)sigma(C-Cl)* state.

Electronic transitions in cis- and trans-dichloroethylenes and tetrachloroethylene.

Electronic structures of trans- and cis-dichloroethylenes and tetrachloroethylene were studied using symmetry-adapted cluster configuration interaction theory. Basis sets up to the aug-cc-pVTZ of Dunning, Jr., augmented with appropriate Rydberg functions were used for the calculations. The results derived in the present study show good agreement with the available experimental values. In all cases, the main bright excitation was the pi-->pi( *) transition. The other vertical excitations, pi-->sigma( *), n-->sigma( *), and n-->pi( *), which have not been studied before, were also investigated. First Rydberg series involving transitions from the pi orbitals to one 3s, three 3p, and five 3d orbitals were identified clearly. Several new assignments and reassignments of features in the experimental spectra were suggested. Contrary to earlier prediction, two n-sigma( *) states, along with a pi-sigma( *) state in the dichloroethylenes, were calculated to be located above the main pi-pi( *) state. Accordingly, crossing between both the n-sigma( *) states with the bright pi-pi( *) state is highly likely, unlike conclusions made in the earlier studies. This indicates that the photodissociation mechanism proposed by the earlier calculations warrants revision. Several low-lying triplet excited states were also studied. Electronic spectra of trans-1-chloro-2-fluoroethylene and cis-1-chloro-2-fluoroethylene were also calculated. The pi-->pi( *) transitions of these haloethylenes are compared and interpreted in terms of the inductive and resonance effects.

Benzdiynes revisited: ab initio and density functional theory

Ab initio and density functional theory (DFT) studies were performed on three isomers of tetradehydrobenzene (benzdiynes). Four different density functionals (BPW91, BLYP, B3LYP, and B1LYP) and two higher levels of theory [QCISD and CCSD(T)] incorporating basis sets up to Dunnings correlation‐consistent polarized valence triple‐ζ (cc‐pVTZ) were utilized for this purpose. Stability tests showed that more stable solutions were available for 1,4‐benzdiyne with unrestricted than with restricted DFT, while solutions obtained with later descriptions of 1,3‐benzdiyne and 1,2,3,5‐tetradehydrobenzene were stable. UB3LYP provided better geometry for 1,4‐benzdiyne. Unlike in an earlier study, the energy difference between 1,2,3,5‐tetradehydrobenzene and 1,4‐benzdiyne calculated with the B3LYP functional coincided well with that calculated at CCSD(T): 13.07 and 14.32 kcal/mol at the B3LYP/cc‐pVTZ and CCSD(T)/cc‐pVTZ levels, respectively. 1,2,3,5‐Tetradehydrobenzene was 8 kcal/mol more stable than 1,3‐benzdiyne at the CCSD(T)/cc‐pVTZ level. The heats of formation of benzdiynes were obtained by using the G2, CBS‐Q, and CBS‐QB3 methodologies. The heats of formation calculated for 1,3‐benzdiyne and 1,2,3,5‐tetradehydrobenzene were 208.6 and 197.9 kcal/mol, respectively, at the CBS‐QB3 level of theory.

Electronic excitations of fluoroethylenes

Several lowest-lying singlet electronic states of vinyl fluoride, trans-, cis-, and 1,1-difluoroethylene, trifluoroethylene, and tetrafluoroethylene were investigated by using symmetry-adapted cluster configuration interaction theory. Basis sets up to Dunnings aug-cc-pVTZ augmented with appropriate Rydberg functions were utilized for the calculations. Calculated excitation energies show a good agreement with the available experimental values. Even in the troublesome pi-->pi(*) transitions, the excitation energies obtained in the present study agree well with the experimental values except in one or two fluoroethylenes. Strong mixing between different states was noticed in a few fluoroethylenes; especially the mixing is very strong between pi-pi(*) and pi-3ppi states in trifluoroethylene. No pure pi-sigma(*) excited state was found in almost all the fluoroethylenes. Several assignments and reassignments of features in the experimental spectra were suggested. The present study does not support the existing argument that the interaction between the pi-pi(*) and sigma-sigma(*) states is the reason behind the blueshift of around 1.25 eV in the pi-pi(*) excitation energy of tetrafluoroethylene. Possible reasons, including structural changes, for this shift are discussed in detail. Several low-lying triplet excited states were also studied.

Chemistry of Cu deposition by Cu(hfac)(tmvs) monitored by Li+ ion attachment mass spectrometry

This work was supported in part by The New Energy and Industrial Technology Development Organization NEDO under a fund for Fundamental Technology Research Facilitation Program.

Origin of the Unusual Ultraviolet Absorption of Arsenicin A

This paper presents a combined experimental and theoretical study of the electronic spectrum of the natural adamantane-type polyarsenical Arsenicin A. Experiments reveal that this molecule strongly absorbs UV light in the absence of an obvious chromophore. The observed absorbance is supported by the time-dependent density functional (TD-DFT) calculations with B3LYP, M06-L, and M06-2X functionals combined with the 6-311+G(3df,2pd) basis set, as well as by symmetry-adapted cluster/configuration interaction (SAC-CI) theory. The theoretical investigations reveal that the absorption is facilitated by through-space and through-bond interactions, between the lone pairs on the arsenic and oxygen atoms and the σ-bonding framework of the molecule, that destabilize occupied and stabilize unoccupied molecular orbitals.

Is the Bisphenol A Biradical Formed in the Pyrolysis of Polycarbonate

An unknown species has been detected in the analysis of the products in a pyrolysis of polycarbonate using Li(+) ion-attachment mass spectrometry (IAMS). The mass spectra exhibited a Li(+) adduct peak at m/z 233 that was tentatively assigned to bisphenol A (BPA) biradical. Experimentally, this assignment was supported by the observation that the production rate increased under an inert nitrogen atmosphere. To further confirm the assignment, the stability of the BPA biradical to intramolecular rearrangement reactions as well as unimolecular decomposition has been analyzed via density functional theory calculations [B3LYP/6-311+G(3df,2p)]. The results show that the bisphenol A biradical is an open-shell biradical singlet that is stable to unimolecular decomposition. Although some of the proposed intramolecular rearrangement products have lower energies than those of the BPA diradical, these pathways have large reaction barriers and the kinetic lifetime of the radical is expected to be of the order of hours under the conditions of the experiment. The calculations also reveal that the bisphenol A diradical has large Li(+) affinities supporting the fact that these Li(+) complexes could be detected in the Li(+) ion attachment mass spectrometry. On the basis of these results the Li(+) adduct peak at m/z 233 detected in the pyrolysis of polycarbonate is assigned to the bisphenol A biradical.

Possible production of C3N4 in the microwave-discharge plasma of C2H2/N2

C3N4 species were unexpectedly detected in an analysis of the products in a C2H2/N2 microwave discharge using Li+ ion-attachment mass spectrometry. The mass spectra exhibited a Li+ adduct peak at a m/z of 99, which was tentatively assigned to C3N4. The possibility of the production of C3N4 was investigated. Discharge conditions were varied with a view toward increasing the production rate and confirming the presence of this compound in the gas phase. The production rate was found to increase with the nitrogen concentration. Both ab initio and density functional theory calculations were also performed.