INDO MO Research Articles

Influence of N‐Substitution on Electron Spin Resonance (ESR) Behavior of 2‐(2‐Nitrophenyl)‐1H‐benzimidazole Derivatives

The synthesis of 2‐(2‐nitrophenyl)‐1H‐benzimidazole (1), 1‐benzoyl‐2‐(2‐nitrophenyl)‐1H ‐benzimidazole (2), and 1‐acetyl‐2‐(2‐nitrophenyl)‐1H‐benzimidazole (3) is reported. Stable radical anions (1 ·−, 2 ·−, and 3 ·−) were generated by chemical reduction in DMSO and studied by ESR spectroscopy. The interpretation of the ESR spectra was done by means of computational simulation process. Hyperfine coupling constants were assigned by comparison with related compounds, and on the basis of calculation based on SCF INDO MO method in the unrestricted Hartree–Fock scheme.

The Generalized Parameterization Procedure for Semiempirical MO Method and Calculation of ionization Potentials for the Fourth‐Period Element Molecules

AbstractThis study presents a set of the generalized parameters from a unique resource for the elements which are from the first‐period to the fourth‐period (H to Xe). In addition, these series of parameters are used to examine forty‐five compounds consisting of the fourth‐period elements and the second‐period transition metal elements. Herein, the major calculation theory applied is the s‐p‐d INDO MO method.To verify the reliability of the molecular calculations, similar calculations of the STO‐3G and STO‐3G* ab inito methods, along with the semiempirical PM3 and AM1 methods are also compared. Results in this study demonstrate that the s‐p‐d INDO MO computed with the generalized parameter is the best available method.

The generalized parameterization procedure for the semiempirical MO method and calculation of ionization potentials for the third row element molecules

The valence electron ionization potentials ( I l ) were collected from a unique resource for the elements from H to Xe. The electron affinities ( A 1) of halogen atoms in each period were obtained from the same resource of I l . The A l values of the remaining elements were obtained by a combined formulation of Slater-Condon integrals and Pariser-Parr approximation. The generalized bond parameter proportionality constant K nl of each period was also evaluated. Using these parameters, twenty-four molecules consisting of only third row elements were selected for the s-p-d INDO-MO calculations. In order to check the reliability of the energy calculations, similar calculations of ab initio type STO-3G ∗, along with semiempirical-type PM3 and AM1 methods were also carried out for comparison. The Koopmans' theorem was applied to compare the experimentally measurement and calculation of the first ionization potentials in all the cases. The results of the present study indicated that the s-p-d INDO MO computed with the new parameterization procedure which given here is the best method among all the methods used in the present work.

Concerning the internal rotational barrier and the experimental and theoretical nJ(13C,13C) and nJ (1H,13C) in ethylbenzene-β-13C

The 1H nuclear magnetic resonance spectra of ethylbenzene-β-13C in CS2/C6D12 and acetone-d6 solutions yield long-range 1H,1H and 1H,13C coupling constants. The 13C {1H} NMR spectra yield 13C, 13C couplings. The conformational dependence of some of these coupling constants is compatible with two values of the barrier to internal rotation about the exocyclic Csp2—Csp3 bond. If the fourfold component of the internal rotational potential is not larger than about 20% of the twofold component and the perpendicular conformer is most stable, then the barrier height is probably less than 6 kJ/mol. However, if the stable conformer has a torsion angle of 60° for the exocyclic C—C bond, then the coupling constants are consistent with a twofold barrier of about 23 kJ/mol. Experimental values of [Formula: see text] and [Formula: see text], where ψ and θ are the torsion angles for the exocyclic C—C and C—H bonds, respectively, are compared to those obtained from INDO MO FPT computations of the angular dependence of nJ(H,C), and nJ(H,H), nJ(C,C) for n ≥ 3. For example, the computations very likely give the correct qualitative ψ dependence of 5J(C,C), yet overestimate its extremum by about a factor of two, either because of an overestimate of the σ–π exchange integrals or because of too large a valence orbital density at the carbon nucleus. Other nJ values are discussed in a similar manner and, because optimized geometries are used in the computations, a somewhat more reliable treatment arises for some coupling constants; an example is 3J(C,C) at small torsion angles.

1H and 13C nuclear magnetic resonance studies of the conformational equilibrium of 2-(diphenylphosphino)benzaldehyde in polar and nonpolar solutions. Signs of the proximate coupling constants, 4J(CHO, 31P) and 3(13CHO, 31P)

The analyses of the 1H nuclear magnetic resonance spectra of 2-(diphenylphosphino)benzaldehyde in CS2/C6D12 and acetone-d6 solutions yield stereospecific coupling constants from which the populations of the O-cis and O-trans conformers are derived. The free energy differences favouring the O-trans conformer at 300 K are 2.7 and 0.9 kJ/mol, in the polar and nonpolar solutions, respectively; in the crystal only the O-cis conformer exists. The coupling constant, 4J(CHO, P), is estimated as −7.1(2) Hz in the O-trans confomer and 3J(CHO, P) as +29.4(1.3) Hz. Their magnitudes depend on the proximity of the C—H bond to the lone pair on phosphorus. nJ(C, P) are reported for triphenylphosphine and for the benzaldehyde derivative as dilute solutions in the two solvents, demonstrating a significant solvent dependence for some of these coupling constants. Some simple relationships are proposed between nJ(C, P) and the torsion angle about the C—P bond, estimates of the latter coming from AM1 and STO 3G MO computations. nJ(C, P) are also sensitive to intrinsic ring substituent perturbations, as are the nJ(H, P); for example, 5J(H, P) is negative in the disubstituted ring of 2-(diphenylphosphino)benzaldehyde but positive in the phenyl groups. The nJ(H, P) are also discussed with respect to their dependence on the torsion angles about the C—P bonds. It appears that the conformational properties of the aromatic rings in triphenylphosphine and its formyl derivative are very similar. Further, the phosphorus atom is polarized such that the carbonyl bond is attracted towards the positive region near phosphorus, and the C—H bond of the formyl group more towards the lone-pair region; the actual torsion angles represent a compromise between these attractive forces and the repulsive forces between bonds on neighbouring aromatic moieties. CNDO/2 MO and INDO MO FPT computations of nJ(C, P) and nJ(H, P) are of mixed utility, although the former bear out the idea that the proximate [Formula: see text]lone-pair interaction dominates 3J(CHO,P) and 4J(CHO,P).

Electron paramagnetic resonance study on some alkylsilane radical cations in γ-irradiated CF3CCl3matrices

Radical cations of ethyl-, propyl-, butyl-, ethylmethyl-, propylmethyl- and diethyl-silanes in CF3CCI3 matrices at 77 K give two-line EPR spectra with separations of around 6 mT, independently of chain lengths and silicon positions. From results on some deuteriated alkylsilanes, and with INDO–MO calculations, it is concluded that each of the two-line spectra is attributable to the hyperfine coupling of the β-trans proton (β-Htrans) with respect to the slightly elongated α-carbon-silicon (Cα–Si) bond, where an unpaired electron is mainly confined. The anti configuration of the Si–Cα–Cβ–H,trans bond sequence is caused by maximization of the hyperconjugative effect between the unpaired electron localized in the Cα–Si bond and the Cβ–Htrans, bond.

DIPOLE MOMENTS OF SOME PHENYL AND CHLOROPHENYL PHOSPHORUS-HALOGEN COMPOUNDS

Abstract The electric dipole moments of o-ClC6H4PCl2 1, C6H5PF4 2, o-ClC6H4PF4 3, C6H5P(O)F2 4, C6H5P(O)HF 5. C6H5P(O)H(OH) 6, and o-ClC6H4P(O)Cl2 7, were determined in dilute benzene solution at 25°C. The phosphoryl bond moment PO in compounds 4, 5, and 7 was estimated using simple bond-additivity calculations. The results obtained were 4.55D, 3.64D, and 5.52D for compounds 4, 5, and 7, respectively. Dipole moments for compounds 2 and 3 were also calculated theoretically using INDO MO techniques. The calculated values differed from the experimental values by about 12%.

Conformational dependence of the one‐bond carbon–proton coupling constants in oligosaccharides

AbstractA study is presented of the dependence on conformation of one‐bond carbon‐proton coupling constants in three 1‐4‐linked disaccharides. Calculated 1J(CH) values are based on the FPT formulation in the semi‐empirical INDO MO method. The configuration at the anomeric carbon influences the 1J(CH) value, and the 1J(C‐1′, H‐1′) values are 20–30 Hz higher than the 1J(C‐4, H‐4) values. The conformational dependence of 1J(CH) on the dihedral angle about the CO bonds is satisfactorily expressed in the analytical form 1J(CH) = A cos 2ϕ + B cos ϕ. The constants A‐E are different for the α‐ and β‐anomers. Calculated average values 〈1J(C‐1′, H‐1′)〉 = 163.7 Hz and 〈1J(C‐4, H‐4)〉 = 146 Hz for methyl β‐xylobioside using PCILO‐calculated abundances of conformers agree well with the experimental values of 162.7 and 147.5 Hz, respectively.

Conformational analysis of methyl phenyl sulphoxides containing fluorine substituents in the phenyl ring based on 1H, 13C and 17O NMR chemical shifts and long‐range nJ(HF) and nJ(CF) coupling constants

AbstractThe preferred conformations of the methylsulphinyl group in a number of phenyl methyl sulphoxides substituted in the phenyl ring with one, two or three fluorine atoms have been studied by employing a multinuclear NMR approach. From the 1H, 13C and 17O chemical shifts of the methylsulphinyl group, and the torsional angles obtained either from MO calculations or experimental determinations for a number of the compounds, empirical correlations have been derived which enabled us to extract the preferred orientation of the SO bond or of the methyl group relative to the phenyl ring in all the molecules examined. For a limited number of the compounds, the structure of the preferred conformer is satisfactorily close to the solid‐state structure found in an independent study carried out with x‐ray diffraction. The twist of the SO bond relative to the phenyl ring changes as a function of the number of ortho substituents, being highest when two substituents are present. The lowest twist, torsional angle nearly zero, was found in ortho‐monosubstituted compounds and the SO bond is oriented towards the opposite side with respect to the substituent. Analyses were also carried out of the conformational dependence of long‐range nJ(HF) and nJ(CF) coupling constants, where the first nucleus in the parentheses refers to the methylsulphinyl group. The behaviour of these coupling constants as a function of internal rotation around the exocyclic CS bond was derived from semi‐empirical INDO MO FPT calculations, and scaled interpolating equations were derived in order to reproduce the experimental results. It thus emerges that 5J(HF), 4J(CF), 5J(CF) and 6J(CF) are conformationally dependent and show, experimentally, significant changes. These parameters can be usefully employed for the conformational analysis of methyl phenyl sulphoxides.

Side-chain conformation and 13C-NMR chemical shift of poly( l-phenylalanine) and oligopeptides containing l-phenylalanine and tyrosine residues in the solid state

The 13C cross-polarization, magic-angle-spinning (CP-MAS) NMR spectra of poly ( l-phenylalanine) and oligopeptides containing a l-phenylalanine or l-tyrosine residue in the solid state have been measured, in order to elucidate the side-chain conformational features of the phenylalanine and tyrosine residues. From these data it was found that the 13C chemical-shift difference between C α (main-chain carbon) and C γ (aromatic carbon) carbons in the l-phenylalanine and l-tyrosine residues in the oligopeptides of which the side-chain conformations in the solid state have already been determined from X-ray diffraction, depends significantly on the side-chain conformation. Using these results, the side-chain conformation of poly( l-phenylalanine) in the α-helix, ω-helix and β-sheet forms was determined. Furthermore, an FPT INDO MO calculation was carried out in order to allow detailed discussion of the side-chain conformation dependence of the 13C chemical shift.

An MO study of the transferability of infrared intensity parameters in alkyl ethers

The transferability of methyl and methylene CH and CO bond polar parameters in dialkyl ethers was studied by calculations employing the INDO MO method. It was found that, in most cases, the local infrared intensity parameters are transferable in the homologous series of molecules. The polarization influence of the electronegative oxygen atom considerably affects parameter values within the first and second neighbouring methylene or methyl groups. Analysis of the electronic charge reorganizations accompanying the vibrational distortions of individual CH bonds show that CH bond polar parameters in these molecules are quite characteristic quantities, strongly correlated with the structure of the respective bonds and groupings.

NMR and indo study of Ni(II) complexes of acetic and propionic acid

The combined use of NMR spin-lattice relaxation and contact shift to investigate the conformational and dynamical properties of Nickel acetate and propionate complexes is reported. The analysis of the C-13 and H-1 contact shift together with INDO MO calculations gave information on the delocalization mechanism and geometries of the complexes. From these geometries, and by means of C-13 and H-1 spin lattice relaxation spectroscopy the correlation time for Ni-ligand dipolar interaction was calculated and found to be identical to the Ni 2+ electron spin lattice relaxation time.

Structure and reaction of methylcyclohexane radical cation: ESR studies

The radical cation of methylcyclohexane (Me-cC 6) is detected by ESR after γ-irradiation at 77 K of a solid solution of the parent molecules in perfluoromethylcyclohexane (CF 3-cC 6F 11) matrix. The isotropic 1H hyperfine (hf) splittings observed are identified with the help of a selectively deuteriated compound and INDO MO calculations: a 1 = 20.1 G (2H), a 2 = 42.7 G (2H), and a 3 = 48.3 G (2H) for the two gauche protons of the methyl group, the equatorial methylene protons at the C 2 and C 6 positions, and those at the C 3 and C 5 positions, respectively, in a chair form with the methyl group at an equatorial position. The unpaired electron in Me-cC 6 + occupies an a″ orbital in C s symmetry. On warming, the radical cation undergoes deprotonation mainly at either the C 3 (or C 5) or C 4 or both.

Long-range formyl proton coupling constants of 4-X-phenyl formats (X = H, F, CH3, NO2) and 2,6-dichlorophenyl formate. Conformations in solution

The signs and magnitudes of the long-range coupling constants between the formyl and ring protons over five, six, seven, and eight bonds in phenyl formate and its 4-fluoro, 4-methyl, and 4-nitro derivatives are reported. These numbers are combined with CNDO/2 and INDO MO FPT computations of their conformational dependence and with STO-3G MO populational distributions to provide a description of the conformational behaviour of these molecules. The Z conformers, predominantly nonplanar and having the carbonyl group pointing towards the ring, appear to account for more than 95% of the molecules in CS2 solution at 300 K. In polar solvents, significant populations of the E conformers are indicated by the long-range coupling constants. The long-range coupling constants for the formyl proton in 2,6-dichlorophenyl formate are consistent with geometry-optimized STO-3G MO computations, which imply that the Z conformation, with the aldehyde group lying near the plane perpendicular to the benzene plane, is again at least 95% abundant at 300 K. The proton chemical shifts are also consistent with these conclusions.

The conformational behaviour of 5Jm(F,CH) in 3,5-difluoro derivatives of toluene, ethylbenzene, and cumene. Applications

The CNDO/2 and INDO MO FPT values for 5Jm(F,CH3) in 3-fluoro- and 3,5-difluorotoluene are exactly reproduced by A cos2 θ + B sin2 θ + C sin2 (θ/2). Here θ is the angle by which the α C—H bond twists out of the benzene plane. Adjustment of A,B, and C to give an agreement with experiment for 3,5-difluorotoluene yields an equation, best considered empirical, which is tested by 5J(F,CH) in 3,5-difluoroethylbenzene and 3,5-difluoroisopropylbenzene. The equation reproduces 5J(F,CH) in these two compounds, the values of [Formula: see text] being derived from 6Jp(H,CH), the spin–spin coupling constants over six bonds between the α and para ring protons. 5J(F,CH) is obtained for the asymmetrical compounds, 2,3-difluorobenzylidene diacetate and 2-bromo-5-fluorobenzylidene diacetate. It is shown how 5J(F,CH) in the latter can discriminate between two conformers, each of which, on the basis of 6J(H,CH), will be characterized by a large degree of torsion about the [Formula: see text] bond.

Structure of radical cations of some methyl-substituted alkanes: ESR and INDO MO studies

Origins of the H 1s spin densities on the trans CH protons were reinvestigated for the radical cations of some methyl-branched small alkanes based on ESR results and INDO MO calculations. It was concluded that the delocalization of spin densities could not be ignored to explain the large and non-equivalent proton hf couplings of the radical cations. The conformations of 2-methylbutane +• and 3-methylpentane +• trapped in CClF 2CCl 2F were found to be different from the previously reported ones.

The effects of intermolecular interactions on 13C NMR chemical shifts. I. Methyl alcohol

Abstract To investigate the effects of possible intermolecular interaction for methyl alcohol in neat liquid, we have measured the 13 C NMR chemical shift of the methyl carbon in 13 C enriched methyl alcohol as a function of pressure up to 500 Kg/cm 2 . The 13 C signal is found to move significantly to high frequency with increasing pressure. This experimental result is reasonably explained by means of sum-over-states INDO and FPT INDO MO theories of nuclear shielding incorporating a modified solvaton theory. From the observed and calculated results the major effect of increasing pressure is predicted to be a decrease in intermolecular distance.

The mechanisms of long-range 13C,19F and 19F,19F coupling constants in derivatives of biphenyl and fluorene. Differential isotope shifts

13C,19F and 19F,19F nuclear spin–spin coupling constants over n formal bonds, n = 1–9, are reported for 4-fluorobiphenyl, 4,4′-difluorobiphenyl, 4,4′-difluoro-2,2′,6,6′-tetramethylbiphenyl, 2,7-difluorofluorene, 2-fluoro-9-fluorenone, and 2,7-difluoro-9-fluorenone in acetone solutions. The signs of many of the coupling constants are deduced from second-order spectral phenomena caused by differential 13C isotope effects on the I9F nmr chemical shifts. Theoretical potentials, based on geometry-optimized STO 3G MO computations for 4-fluorobiphenyl and 4,4′-difluorobiphenyl, yield expectation values for the torsion angles about the exocyclic C—C linkage that are very close to those deduced from electron diffraction patterns. These potentials and INDO MO FPT computations of the long-range coupling constants allow a discussion of the coupling mechanisms. In Hz, 9J(F,F) = 1.3(1) cos2 θ, where θ is zero for a planar biphenyl, while 8J(C,F) = 0.8(1) cos2 θ and 7J(C,F) = −0.43(5) cos2 θ. 6J(C,F) is a composite of σ–π and π electron coupling components and is written in Hz as 0.57(1) + 0.29(1) sin2 θ. The corresponding coupling constants in the fluorene and 9-fluorenone derivatives are enhanced in magnitude relative to a hypothetical planar biphenyl derivative. It is tentatively suggested that 5J(C,F) consists of three coupling components, one negative and proportional to cos2 θ, the other two positive and independent of θ. 4J(C,F) is suggested to consist of a σ component of −1.0 Hz and a π component proportional to the atom–atom polarizability for the parent hydrocarbon.

Strength of the metal-ligand bonds in some bicyclopentadienyl and dibenzene compounds of 3d transition elements

1. The mean values of the “internal” metal-ligand bond dissociation energies ¯D1(M-L) and Wiberg indices calculated by the INDO MO method W(M-L) are virtually invariant within the series of dicyclopentadienyl and dibenzene π-complexes of 3d transition metals. 2. The similarity in the behavior of the ¯D1(M-L) values and the W(M-L) values which characterize covalent bonding indicates that the contribution of the ionic component to the metal-ligand bond is either small or approximately identical in each of these series of compounds.

Conformational preference of substituted naphthalenes—IV. 2-Hydroxy-1-naphthaldehyde

The 100 MHz spectrum of 2-hydroxy-1-naphthaldehyde dissolved in carbon tetrachloride was analysed. The long-range coupling constants from the aromatic protons to those in the aldehyde and hydroxyl groups were compared with values calculated by INDO MO FPT methods. The results confirm the view that the predominant conformation has a hydrogen bond between the oxygen of the aldehyde group and the hydroxyl proton.