Varsha S H R I D H A R Gejji (original) (raw)
Papers by Varsha S H R I D H A R Gejji
![Research paper thumbnail of Encapsulation of alkyl and aryl derivatives of quaternary ammonium cations within cucurbit[n]uril (n = 6,7) and their inverted diastereomers: density functional investigations](https://attachments.academia-assets.com/69418089/thumbnails/1.jpg)
](Journal of Molecular Modeling, 2014
Electronic structure, vibrational frequencies and 1 H chemical shifts of inclusion complexes betw... more Electronic structure, vibrational frequencies and 1 H chemical shifts of inclusion complexes between CB[n] (n=6,7) or their inverted iCB[n] diastereomer hosts and quaternary diammonium viz., 1,6-hexyldiammonium (HDA) or p-xylyldiammonium (XYL) cationic guests are obtained from the density functional calculations. The interaction of CB[n] or iCB[n] with HDA (guest) conduce inclusion complexes in which the guest attains gauche conformation within the host cavity. The lowest energy XYL complexes of CB[6] or iCB[6] are comprised of one ammonium group orienting parallel to aromatic ring. The CB[7] or iCB[7] complexes of XYL on the other hand, reveal ammonium group(s) perpendicular to aromatic ring of the guest. The ureido C=O and N-H stretching vibrations on complexation engender frequency down-shift in the calculated spectra. This can be attributed to C-H-O and N-H-O interactions in the complex. The inverting of glycouril unit in iCB[n] renders a frequency shift (12 cm-1) for the C=O stretching in the opposite direction. Molecular electron density topography and natural bond orbital analyses have been used to explain the direction of frequency shifts. Calculated 1 H NMR reveal that guest protons within the host cavity not participating in hydrogen bonding interactions, exhibit shielded signals compared to isolated XYL or HDA. Likewise the inverted protons in the iCB[6]-XYL complex led to up-field signals in calculated 1 H NMR as a result of C-H-π interactions.
Journal of Physical Chemistry A, 1997
The topography of molecular electrostatic potential (MESP) has been proposed as a predictive tool... more The topography of molecular electrostatic potential (MESP) has been proposed as a predictive tool for studying ion-pair formation, using the trifluromethanesulfonate (triflate) ion, CF3SO3-(Tf-), as a test example. A model based on electrostatic docking of the cation (Li+ ...
Chemical Physics Letters, 2001
The trends for cation binding for several conformers of diglyme are predicted by mapping the topo... more The trends for cation binding for several conformers of diglyme are predicted by mapping the topography of the molecular electrostatic potential (MESP) at the Hartree±Fock (HF) level. Dierent Li ±diglyme geometries derived by exploiting the MESP cooperative eects are used subsequently in ab initio computations. The binding energies for Li with diglyme have been calculated in mono-, bi-and tridentate coordinations by employing the HF, second-order Mùller±Plesset (MP2) and the hybrid density functional methods. The calculated vibrational spectrum of Li ±diglyme also points to a gauche conformation of diglyme in the complex. Ó
Physical Review A, 1982
The <r/sup n/> expe... more The <r/sup n/> expectation values have been extracted from the known electron momentum distributions for hydrogen, helium, argon, and krypton atoms with the use of a semiclassical approach. These values compare fairly well with their Hartree-Fock counterparts. This procedure provides a link between the distribution of electrons in the momentum space and that in the coordinate space.
Chemical Physics Letters, 1984
ABSTRACT
Journal of Physical Chemistry A, 2007
The influence of a uniform static external electric field on some aliphatic and aromatic molecula... more The influence of a uniform static external electric field on some aliphatic and aromatic molecular species is studied within the density functional theory (DFT) employing the 6-311++G(2d,2p) basis set with B3LYP exchange-correlation prescription. The electric field perturbs the molecular geometry but drastically alters the dipole moments and engenders, to a varying degree, the molecular vibrational Stark effect, i.e., shifts in the infrared (IR) vibrational frequencies accompanied by spectral intensity redistribution. For polar molecules, significant negative ("red") and positive ("blue") frequency shifts are observed for field orientations both parallel and antiparallel to their permanent dipole moments. Further, a selective reordering of frontier orbitals is observed to be brought about by moderately intense fields. In particular, molecules having a lowest unoccupied molecular orbital (LUMO) with predominant π character possess a threshold field beyond which energy gap between the highest occupied molecular orbital (HOMO) and LUMO diminishes rapidly. A timedependent (TD) DFT analysis reveals that an increase in the applied field strength by and large increases the excitation energies corresponding to significant electronic transitions among frontier MOs with a concomitant decrease in their oscillator strengths.
Journal of Physical Chemistry A, 2007
The influence of a uniform static external electric field on some aliphatic and aromatic molecula... more The influence of a uniform static external electric field on some aliphatic and aromatic molecular species is studied within the density functional theory (DFT) employing the 6-311++G(2d,2p) basis set with B3LYP exchange-correlation prescription. The electric field perturbs the molecular geometry but drastically alters the dipole moments and engenders, to a varying degree, the molecular vibrational Stark effect, i.e., shifts in the infrared (IR) vibrational frequencies accompanied by spectral intensity redistribution. For polar molecules, significant negative ("red") and positive ("blue") frequency shifts are observed for field orientations both parallel and antiparallel to their permanent dipole moments. Further, a selective reordering of frontier orbitals is observed to be brought about by moderately intense fields. In particular, molecules having a lowest unoccupied molecular orbital (LUMO) with predominant π character possess a threshold field beyond which energy gap between the highest occupied molecular orbital (HOMO) and LUMO diminishes rapidly. A timedependent (TD) DFT analysis reveals that an increase in the applied field strength by and large increases the excitation energies corresponding to significant electronic transitions among frontier MOs with a concomitant decrease in their oscillator strengths.
The Journal of Physical Chemistry, 1990
... Potasslum Hydrogen Diformate, Including Electron Correlation Shridhar P. Gejji, Oscar E. Taur... more ... Potasslum Hydrogen Diformate, Including Electron Correlation Shridhar P. Gejji, Oscar E. Taurian,+ and Sten Lunell* ... 1934, 46, 618. (!4) Frisch, MJ; Binkley, J. S.; Schlegel, H. B.; Raghavachari, K.; Mel~us, CF: Martin, RL; Stewart, JJ P.; Bobrowicz, FW; Rohlfing, c. M.; Kahn, L. R ...
Journal of Physical Chemistry A, 1999
The cation coordination of the bis(trifluoromethanesulfone)imide (TFSI) anion (CF 3 SO 2 ) 2 N - ... more The cation coordination of the bis(trifluoromethanesulfone)imide (TFSI) anion (CF 3 SO 2 ) 2 N - has been studied using the molecular electrostatic potential (MESP) topography as a tool. The critical points of MESP are employed for searching the cation binding sites of the ...
Journal of Molecular Structure-theochem, 2005
Ab initio Hartree±Fock and the hybrid density functional theoretical investigations on the struct... more Ab initio Hartree±Fock and the hybrid density functional theoretical investigations on the structure and vibrational frequencies of Schiff's base ligand 1-(salicylaldimino)-3-biuret (1) and its anion are carried out by using the Dunning-Huzinaga double zeta basis set. For 1 the syn and anti conformers, both having intramolecular hydrogen bonding, are obtained as the local minima on the potential energy surface. In the syn conformer, the hydrogen bond through nitrogen of imine group results in a formation of a six membered ring which stabilizes this conformer over the anti conformer. The energy difference of the syn and anti conformers from the hybrid density functional theory has been predicted to be 28.09 kJ mol 21 . A comparison of the vibrational spectra of the anion and the syn conformer from the density functional theory predicts that both the 1763 and 1715 cm 21 carbonyl stretching vibrations are up-shifted by ,50 cm 21 on protonation of the anion. Also the new bands at 1576 and 1326 cm 21 assigned to HNH scissoring and strongly coupled CN stretching with CCH rocking, respectively, will appear. The 1326 cm 21 vibration is, however, predicted only for the more stable syn conformer. q 2. Computational method gaussian 94 quantum chemistry package [10] was Journal of Molecular Structure (Theochem) 585 (2002) 159±165 0166-1280/02/$ -see front matter q
Physical Review A, 1983
Electron momentum distributions have been obtained from electron densities derived from the Thoma... more Electron momentum distributions have been obtained from electron densities derived from the Thomas-Fermi, local-density-functional, and general-density-functional theories by the application of the procedure of Burkhardt, Konya, and Coulson and March (the BKCM procedure). The BKCM procedure has been modified so as to simulate the correct asymptotic behavior of the electron momentum distributions. It has been shown that, by means of reciprocal relationships to those in the direct BKCM procedure for estimating electron momentum density' from a given electron density, a reverse transformation from electron momentum density to electron density can be effected.
Chemical Physics Letters, 2004
Hartree-Fock and density functional methods have been used to obtain the molecular structure, ele... more Hartree-Fock and density functional methods have been used to obtain the molecular structure, electron density topography and the vibrational frequencies of trimethylammoiumhexafluorosilicate (TMAHFS). In the isolated molecule the N-HÁ Á ÁF bond is predicted to be nonlinear with a deviation of 16°from linearity and the SiF 2À 6 orients with a dihedral angle of 27°relative to the trimethylammonium cation. Blue shifts of 66 and 45 cm À1 for the Si-F and C-N stretchings, respectively, compared to the respective vibrations in the free anion and cation, predicted from the density functional calculations have been explained from the difference electron density and bond critical points in electron density topography.
Physical Review A, 1984
The scope of the quasiclassical procedure previously used by us for estimating atomic-electron mo... more The scope of the quasiclassical procedure previously used by us for estimating atomic-electron momentum densities exclusively from the knowledge of electron densities has been extended to diatomic molecules. This procedure yields a spherically averaged molecular-electron ...
Journal of Molecular Structure-theochem, 2005
Molecular electrostatic potential and Electron density topography in a series of nitrotriprismane... more Molecular electrostatic potential and Electron density topography in a series of nitrotriprismanes (C 6 H 6Ka (NO 2 ) a , aZ1-6) have been analyzed using the Hartree-Fock and hybrid density functional methods. Weak, electrostatic C-H/O interactions, which render more stability to the isomers of nitroprismane, manifest in the shallow minima of the nitro groups along a series. As revealed from the molecular electron density topography the critical points of the X-N (XZC or N) bonds correlate well with the heat of formation derived from the isodesmic reaction approach. q Journal of Molecular Structure: THEOCHEM 724 (2005) 87-93 www.elsevier.com/locate/theochem 0166-1280/$ -see front matter q
Theoretical Chemistry Accounts, 2004
Energetics and the charge distributions in azacubanes (C8NαH8−α) have been obtained using the ab ... more Energetics and the charge distributions in azacubanes (C8NαH8−α) have been obtained using the ab initio Hartree–Fock, second-order Mø øller–Plesset perturbation theory and hybrid density functional methods. For diazacubane to hexaazacubane the lowest-energy conformers have nitrogen atoms occupying the face opposite corners of a cube. The topography of the molecular electrostatic potential and the electron density of azacubane conformers have been investigated. The electrostatic potential studies have shown that successive substitution of nitrogen instead of CH groups of cubane engenders smaller and more localized electron-rich regions around the nitrogens of a cube. Further the bond ellipticity and the electron density at the bond critical point of the X–N bonds (X=C or N) in a cubanoid increase from azacubane to octaazacubane. The heats of formation of azacubanes calculated by the isodesmic reaction approach using different levels of theory correlate well with the electron density at the bond critical point of X–N (X=C or N) bonds in a cubanoid.
Journal of Molecular Structure-theochem, 2003
Structures of the phenol-imine and quinoid tautomers of the titled ligand have been obtained from... more Structures of the phenol-imine and quinoid tautomers of the titled ligand have been obtained from the ab initio and hybrid density functional methods. The phenol-imine tautomer having bifurcated N⋯H hydrogen bonds (1.799 and 1.833Å), has been predicted to be 14.9 kJ mol−1 more stable than the quinoid tautomer wherein O⋯H hydrogen bonded interactions are present. Charge distributions in these tautomers
Journal of Molecular Structure-theochem, 2004
Molecular electrostatic potential and Electron density topography in a series of nitrotriprismane... more Molecular electrostatic potential and Electron density topography in a series of nitrotriprismanes (C 6 H 6Ka (NO 2 ) a , aZ1-6) have been analyzed using the Hartree-Fock and hybrid density functional methods. Weak, electrostatic C-H/O interactions, which render more stability to the isomers of nitroprismane, manifest in the shallow minima of the nitro groups along a series. As revealed from the molecular electron density topography the critical points of the X-N (XZC or N) bonds correlate well with the heat of formation derived from the isodesmic reaction approach. q Journal of Molecular Structure: THEOCHEM 724 (2005) 87-93 www.elsevier.com/locate/theochem 0166-1280/$ -see front matter q
Journal of Molecular Modeling, 2014
Electronic structure, vibrational frequencies and 1 H chemical shifts of inclusion complexes betw... more Electronic structure, vibrational frequencies and 1 H chemical shifts of inclusion complexes between CB[n] (n=6,7) or their inverted iCB[n] diastereomer hosts and quaternary diammonium viz., 1,6-hexyldiammonium (HDA) or p-xylyldiammonium (XYL) cationic guests are obtained from the density functional calculations. The interaction of CB[n] or iCB[n] with HDA (guest) conduce inclusion complexes in which the guest attains gauche conformation within the host cavity. The lowest energy XYL complexes of CB[6] or iCB[6] are comprised of one ammonium group orienting parallel to aromatic ring. The CB[7] or iCB[7] complexes of XYL on the other hand, reveal ammonium group(s) perpendicular to aromatic ring of the guest. The ureido C=O and N-H stretching vibrations on complexation engender frequency down-shift in the calculated spectra. This can be attributed to C-H-O and N-H-O interactions in the complex. The inverting of glycouril unit in iCB[n] renders a frequency shift (12 cm-1) for the C=O stretching in the opposite direction. Molecular electron density topography and natural bond orbital analyses have been used to explain the direction of frequency shifts. Calculated 1 H NMR reveal that guest protons within the host cavity not participating in hydrogen bonding interactions, exhibit shielded signals compared to isolated XYL or HDA. Likewise the inverted protons in the iCB[6]-XYL complex led to up-field signals in calculated 1 H NMR as a result of C-H-π interactions.
Journal of Physical Chemistry A, 1997
The topography of molecular electrostatic potential (MESP) has been proposed as a predictive tool... more The topography of molecular electrostatic potential (MESP) has been proposed as a predictive tool for studying ion-pair formation, using the trifluromethanesulfonate (triflate) ion, CF3SO3-(Tf-), as a test example. A model based on electrostatic docking of the cation (Li+ ...
Chemical Physics Letters, 2001
The trends for cation binding for several conformers of diglyme are predicted by mapping the topo... more The trends for cation binding for several conformers of diglyme are predicted by mapping the topography of the molecular electrostatic potential (MESP) at the Hartree±Fock (HF) level. Dierent Li ±diglyme geometries derived by exploiting the MESP cooperative eects are used subsequently in ab initio computations. The binding energies for Li with diglyme have been calculated in mono-, bi-and tridentate coordinations by employing the HF, second-order Mùller±Plesset (MP2) and the hybrid density functional methods. The calculated vibrational spectrum of Li ±diglyme also points to a gauche conformation of diglyme in the complex. Ó
Physical Review A, 1982
The <r/sup n/> expe... more The <r/sup n/> expectation values have been extracted from the known electron momentum distributions for hydrogen, helium, argon, and krypton atoms with the use of a semiclassical approach. These values compare fairly well with their Hartree-Fock counterparts. This procedure provides a link between the distribution of electrons in the momentum space and that in the coordinate space.
Chemical Physics Letters, 1984
ABSTRACT
Journal of Physical Chemistry A, 2007
The influence of a uniform static external electric field on some aliphatic and aromatic molecula... more The influence of a uniform static external electric field on some aliphatic and aromatic molecular species is studied within the density functional theory (DFT) employing the 6-311++G(2d,2p) basis set with B3LYP exchange-correlation prescription. The electric field perturbs the molecular geometry but drastically alters the dipole moments and engenders, to a varying degree, the molecular vibrational Stark effect, i.e., shifts in the infrared (IR) vibrational frequencies accompanied by spectral intensity redistribution. For polar molecules, significant negative ("red") and positive ("blue") frequency shifts are observed for field orientations both parallel and antiparallel to their permanent dipole moments. Further, a selective reordering of frontier orbitals is observed to be brought about by moderately intense fields. In particular, molecules having a lowest unoccupied molecular orbital (LUMO) with predominant π character possess a threshold field beyond which energy gap between the highest occupied molecular orbital (HOMO) and LUMO diminishes rapidly. A timedependent (TD) DFT analysis reveals that an increase in the applied field strength by and large increases the excitation energies corresponding to significant electronic transitions among frontier MOs with a concomitant decrease in their oscillator strengths.
Journal of Physical Chemistry A, 2007
The influence of a uniform static external electric field on some aliphatic and aromatic molecula... more The influence of a uniform static external electric field on some aliphatic and aromatic molecular species is studied within the density functional theory (DFT) employing the 6-311++G(2d,2p) basis set with B3LYP exchange-correlation prescription. The electric field perturbs the molecular geometry but drastically alters the dipole moments and engenders, to a varying degree, the molecular vibrational Stark effect, i.e., shifts in the infrared (IR) vibrational frequencies accompanied by spectral intensity redistribution. For polar molecules, significant negative ("red") and positive ("blue") frequency shifts are observed for field orientations both parallel and antiparallel to their permanent dipole moments. Further, a selective reordering of frontier orbitals is observed to be brought about by moderately intense fields. In particular, molecules having a lowest unoccupied molecular orbital (LUMO) with predominant π character possess a threshold field beyond which energy gap between the highest occupied molecular orbital (HOMO) and LUMO diminishes rapidly. A timedependent (TD) DFT analysis reveals that an increase in the applied field strength by and large increases the excitation energies corresponding to significant electronic transitions among frontier MOs with a concomitant decrease in their oscillator strengths.
The Journal of Physical Chemistry, 1990
... Potasslum Hydrogen Diformate, Including Electron Correlation Shridhar P. Gejji, Oscar E. Taur... more ... Potasslum Hydrogen Diformate, Including Electron Correlation Shridhar P. Gejji, Oscar E. Taurian,+ and Sten Lunell* ... 1934, 46, 618. (!4) Frisch, MJ; Binkley, J. S.; Schlegel, H. B.; Raghavachari, K.; Mel~us, CF: Martin, RL; Stewart, JJ P.; Bobrowicz, FW; Rohlfing, c. M.; Kahn, L. R ...
Journal of Physical Chemistry A, 1999
The cation coordination of the bis(trifluoromethanesulfone)imide (TFSI) anion (CF 3 SO 2 ) 2 N - ... more The cation coordination of the bis(trifluoromethanesulfone)imide (TFSI) anion (CF 3 SO 2 ) 2 N - has been studied using the molecular electrostatic potential (MESP) topography as a tool. The critical points of MESP are employed for searching the cation binding sites of the ...
Journal of Molecular Structure-theochem, 2005
Ab initio Hartree±Fock and the hybrid density functional theoretical investigations on the struct... more Ab initio Hartree±Fock and the hybrid density functional theoretical investigations on the structure and vibrational frequencies of Schiff's base ligand 1-(salicylaldimino)-3-biuret (1) and its anion are carried out by using the Dunning-Huzinaga double zeta basis set. For 1 the syn and anti conformers, both having intramolecular hydrogen bonding, are obtained as the local minima on the potential energy surface. In the syn conformer, the hydrogen bond through nitrogen of imine group results in a formation of a six membered ring which stabilizes this conformer over the anti conformer. The energy difference of the syn and anti conformers from the hybrid density functional theory has been predicted to be 28.09 kJ mol 21 . A comparison of the vibrational spectra of the anion and the syn conformer from the density functional theory predicts that both the 1763 and 1715 cm 21 carbonyl stretching vibrations are up-shifted by ,50 cm 21 on protonation of the anion. Also the new bands at 1576 and 1326 cm 21 assigned to HNH scissoring and strongly coupled CN stretching with CCH rocking, respectively, will appear. The 1326 cm 21 vibration is, however, predicted only for the more stable syn conformer. q 2. Computational method gaussian 94 quantum chemistry package [10] was Journal of Molecular Structure (Theochem) 585 (2002) 159±165 0166-1280/02/$ -see front matter q
Physical Review A, 1983
Electron momentum distributions have been obtained from electron densities derived from the Thoma... more Electron momentum distributions have been obtained from electron densities derived from the Thomas-Fermi, local-density-functional, and general-density-functional theories by the application of the procedure of Burkhardt, Konya, and Coulson and March (the BKCM procedure). The BKCM procedure has been modified so as to simulate the correct asymptotic behavior of the electron momentum distributions. It has been shown that, by means of reciprocal relationships to those in the direct BKCM procedure for estimating electron momentum density' from a given electron density, a reverse transformation from electron momentum density to electron density can be effected.
Chemical Physics Letters, 2004
Hartree-Fock and density functional methods have been used to obtain the molecular structure, ele... more Hartree-Fock and density functional methods have been used to obtain the molecular structure, electron density topography and the vibrational frequencies of trimethylammoiumhexafluorosilicate (TMAHFS). In the isolated molecule the N-HÁ Á ÁF bond is predicted to be nonlinear with a deviation of 16°from linearity and the SiF 2À 6 orients with a dihedral angle of 27°relative to the trimethylammonium cation. Blue shifts of 66 and 45 cm À1 for the Si-F and C-N stretchings, respectively, compared to the respective vibrations in the free anion and cation, predicted from the density functional calculations have been explained from the difference electron density and bond critical points in electron density topography.
Physical Review A, 1984
The scope of the quasiclassical procedure previously used by us for estimating atomic-electron mo... more The scope of the quasiclassical procedure previously used by us for estimating atomic-electron momentum densities exclusively from the knowledge of electron densities has been extended to diatomic molecules. This procedure yields a spherically averaged molecular-electron ...
Journal of Molecular Structure-theochem, 2005
Molecular electrostatic potential and Electron density topography in a series of nitrotriprismane... more Molecular electrostatic potential and Electron density topography in a series of nitrotriprismanes (C 6 H 6Ka (NO 2 ) a , aZ1-6) have been analyzed using the Hartree-Fock and hybrid density functional methods. Weak, electrostatic C-H/O interactions, which render more stability to the isomers of nitroprismane, manifest in the shallow minima of the nitro groups along a series. As revealed from the molecular electron density topography the critical points of the X-N (XZC or N) bonds correlate well with the heat of formation derived from the isodesmic reaction approach. q Journal of Molecular Structure: THEOCHEM 724 (2005) 87-93 www.elsevier.com/locate/theochem 0166-1280/$ -see front matter q
Theoretical Chemistry Accounts, 2004
Energetics and the charge distributions in azacubanes (C8NαH8−α) have been obtained using the ab ... more Energetics and the charge distributions in azacubanes (C8NαH8−α) have been obtained using the ab initio Hartree–Fock, second-order Mø øller–Plesset perturbation theory and hybrid density functional methods. For diazacubane to hexaazacubane the lowest-energy conformers have nitrogen atoms occupying the face opposite corners of a cube. The topography of the molecular electrostatic potential and the electron density of azacubane conformers have been investigated. The electrostatic potential studies have shown that successive substitution of nitrogen instead of CH groups of cubane engenders smaller and more localized electron-rich regions around the nitrogens of a cube. Further the bond ellipticity and the electron density at the bond critical point of the X–N bonds (X=C or N) in a cubanoid increase from azacubane to octaazacubane. The heats of formation of azacubanes calculated by the isodesmic reaction approach using different levels of theory correlate well with the electron density at the bond critical point of X–N (X=C or N) bonds in a cubanoid.
Journal of Molecular Structure-theochem, 2003
Structures of the phenol-imine and quinoid tautomers of the titled ligand have been obtained from... more Structures of the phenol-imine and quinoid tautomers of the titled ligand have been obtained from the ab initio and hybrid density functional methods. The phenol-imine tautomer having bifurcated N⋯H hydrogen bonds (1.799 and 1.833Å), has been predicted to be 14.9 kJ mol−1 more stable than the quinoid tautomer wherein O⋯H hydrogen bonded interactions are present. Charge distributions in these tautomers
Journal of Molecular Structure-theochem, 2004
Molecular electrostatic potential and Electron density topography in a series of nitrotriprismane... more Molecular electrostatic potential and Electron density topography in a series of nitrotriprismanes (C 6 H 6Ka (NO 2 ) a , aZ1-6) have been analyzed using the Hartree-Fock and hybrid density functional methods. Weak, electrostatic C-H/O interactions, which render more stability to the isomers of nitroprismane, manifest in the shallow minima of the nitro groups along a series. As revealed from the molecular electron density topography the critical points of the X-N (XZC or N) bonds correlate well with the heat of formation derived from the isodesmic reaction approach. q Journal of Molecular Structure: THEOCHEM 724 (2005) 87-93 www.elsevier.com/locate/theochem 0166-1280/$ -see front matter q