Coupled cluster and DFT calculations of 14N nuclear quadrupole coupling constants

The dependence of ¹⁴N quadrupole coupling constants calculated using coupled cluster theory on the level of approximation is examined for a series of small molecules. For HCN, HNC, CH₃CN, and CH₃NC, we use the coupled cluster singles‐and‐doubles with a noniterative perturbative triples correction—CCSD(T)—approach, and we analyze the basis set dependence of the results. For aziridine, diazirine, and cyclopropyl cyanide, we use the CCSD(T) approach, but smaller basis sets, and for the largest studied molecules—quinuclidine and hexamine—we present CCSD results. The differences between computed and experimental values for the best basis sets used are ≈ 5% at the CCSD level and decrease noticeably at the CCSD(T) level. The ‐ N≡C bonds are an exception—in this case the quadrupole coupling constants are very small, hence the differences between theory and experiment become larger (up to 9%). We also consider the performance of density functional theory, comparing the results for different density functionals with the coupled cluster values of the same constants. Most of the functionals provide results systematically improved with respect to the Hartree–Fock values, with ¹⁴N coupling constants in ‐ N≡C bonds being again an exception. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Force constants,vibrational assignment and geometry of methyl amine from hartree—fock calculations

The force constants and geometry of CH₃NH₂ have been calculated from Hartree—Fock wave-functions by the force method, using a 73/3/1 Gaussian basis set. The fundamental frequencies obtained from the ab initio force constants corroborate the assignment of Gray and Lord except for the uncertain A″ NH₂ twisting and CH₃ rocking frequencies. The results indicate that the 1335 cm^?1 band in CH₃NH₂ is v₁₃, the antisymmetric combination of these modes, and that their symmetric combination, v₁₄, is located between 880 and 1000 cm^?1. The calculations reproduce the experimentally observed tilting of the CH₃ group toward the lone pair on nitrogen.     

The microwave spectrum,structure and centrifugal distortion constants of 2-chloroacrylonitrile

The microwave spectrum of 2-chloroacrylonitrile has been studied in the 26.5–40 GHz region. A total of 99 a- and b-type rotational transitions have been measured and assigned for CH₂ =C³⁵Cl(CN),yielding values for the rotational constants (in MHz): A = 6973.27, B = 3148.16, C = 2165.95. For CH₂=C³⁷Cl(CN) a total of 53 transitions have been measured and assigned and the rotational constants obtained are (in MHz): A = 6909.35, B = 3081.17, C = 2127.98. The distortion effects have also been studied and the quartic distortion constants have been evaluated. From the observed hyperfine structure, the chlorine nuclear quadrupole coupling constants have been obtained. The structure of vinyl cyanide and vinyl chloride can be transferred to account remarkably well for the observed rotational constants.     

An ab initio molecular orbital study of the deprotonation of amines

The geometries of the amines NH₂X and amido anions NHX^?, where X = H, CH₃, NH₂, OH, F, C₂H, CHO, and CN have been optimized using ab initio molecular orbital calculations with a 4-31G basis set. The profiles to rotation about the N? X bonds in CH₃NH^?, NH₂NH^?, and HONH^? are very similar to those for the isoprotic and isoelectronic neutral compounds CH₃OH, NH₂OH, and HOOH. The amines with unsaturated bonds adjacent to the nitrogen atoms undergo considerable skeletal rearrangement on deprotonation such that most of the negative charge of the anion is on the substituent. The computed order of acidity for the amines NH₂X is X = CN > HCO > F ≈ C₂H > OH > NH₂ > CH₃ > H and for the reaction NHX^? + H⁺ → NH₂X the computed energies vary over the range 373–438 kcal/mol.     

A perchlorate-promoted electrochemically induced nitride coupling reaction

The osmium nitride complex [Os^VI(NH₃)₄N]³⁺ undergoes a one-electron reduction in acetonitrile to give [Os^V(N)(NH₃)₄]²⁺, which further reacts by nitride coupling to give the μ-dinitrogen osmium complex [(CH₃CN)(NH₃)₄Os^II(N₂)Os^II(NH₃)₄(CH₃CN)]⁴⁺. The formation of the μ-dinitrogen osmium complex is promoted by the presence of perchlorate anion, which causes the deposition of [(CH₃CN)(NH₃)₄Os^II(N₂)Os^II(NH₃)₄(CH₃CN)](ClO₄)₄ on the electrode surface upon repetitive voltammetric scans.     

Localized molecular orbital studies in momentum space. II. Correspondence between coordinate and momentum space properties of various electron pairs

The momentum space properties of the ten-electron systems Ne, HF, H₂O, NH₃ and CH₄ as well as those of CH₃CH₃, CH₃NH₂, CH₃OH and FCH₂OH were investigated using localized molecular orbitals (LMO) obtained from ab initio self-consistent-field (SCF) wavefunctions constructed from double zeta quality gaussain basis sets.Compton profiles of various LMO electron pairs (CC, CN, CO, CF; CH, NH, OH, FH bond pairs and C, N, O, F lone pairs) are tabulated. In order to understand the correspondence between the momentum and the coordinate space properties of those electron pairs, the concept of the size and the shape of an LMO electron pair charge distribution has been utilized. The use of the intermediate expectation values of pⁿ is introduced for the purpose of interpreting the momentum space properties.The dependence of molecular property partitioning on different localization schemes and on different basis sets is also studied by using the H₂O profile as an example.     

subject index

Two conformers of piperidine interconvert through nitrogen inversion. In this equilibrium, conformers with equatorial N–H appear dominant over those with axial N–H. In this study, substituent (X) effects are investigated on nitrogen inversions, for axially substituted 3-X-piperidines, using ab initio calculated NQR parameters, where X=H, F, Cl, Br, CH₃, CF₃, OH, CHO, COOH, CN, NH₂ and NO₂. These 12 species are in three forms. They may have an axial sp³ N–H (1_X), a planar sp² N–H (2_X), and/or an equatorial sp³ N–H (3_X). The axial minima, 1_X, appear to go through transition states (TS), 2_X, and convert to equatorial minima, 3_X. Ab initio and DFT methods are employed to study these equilibria (1_X (min)⇌2_X(TS)⇌3_X(min)). Energies, structural parameters as well as ²H (²H–N) and ¹⁴N quadrupole coupling constants (χ_2H and χ_14N, respectively) are calculated for 1_X, 2_X and 3_X. The order of χ_2H magnitude appears to be: 2_X>3_X>1_X. The same order is obtained for χ_14N when X=F, Cl, Br, CH₃, CF₃, COOH, CN, NH₂ and NO₂. For X=H, OH and CHO, χ_14N trend changes to: 2_X>1_X>3_X. According to our B3LYP/6-31 g (d,p) and MP2/6-31g(d,p) data, the equilibrium shifts in favor of 1_X over 3_X form is demonstrated, when X is F, Cl, Br, COOH, NO₂, CN or CF₃. On the other hand, equatorial 3_X dominate over axial 1_X when X=H, CH₃, OH and NH₂.     

Theoretical study of N quadrupole coupling constants in some NO-containing complexes: N2O3 and FNO

The nuclear quadrupole coupling constants at nitrogen centers have been computed for N₂O₃ and FNO by employing the complete-active-space self-consistent field, internally contracted multireference configuration interaction and single-configuration coupled-cluster methods with correlation-consistent basis sets at the levels of attainable accuracy. To examine the overall quality of the wave functions used in our calculations, also electric dipole moments and potential energy characteristics were calculated and compared with available experimental and recent theoretical data. The effects of the choice of the basis set and reference configuration space were investigated. The robust changes in the electric field gradients occurring in the course of complex formation from isolated subunits were interpreted in terms of wave function composition. Our calculations confirm the assignment of the ¹⁴N nuclear quadrupole coupling constants to nuclear centers in N₂O₃ provided by the microwave measurements of Cox et al. [A.P. Cox, J. Randell, A.C. Legon, Chem. Phys. Lett. 126 (1986) 481.].     

Novel ring transformations of pyrazines by intramolecular diels-alder reactions

Pyrazines carrying the ω-alkyne side-chain -XCH₂CH₂C≡CH, (X = 0,N,S,SO,SO₂, C(CN)₂) undergo on heating an intramolecular Diels-Alder reaction. Pyrazines with the electron donating atom (0,N or S) in the side-chain afford [c]-fused pyridines as main products, whereas (3-buty-nylsulfinyl)pyrazine and (3-butynylsulfonyl)pyrazine are exclusively converted into [b]-fused pyridines. A [b]-fused pyridine is also the major product in the reaction of 2,5-bis(1,1-dicyano-4-pentynyl)pyrazine.     

Ab initio calculations of isotropic hyperfine coupling constants in β-ketoenolyl radicals

Ab initio unrestricted Hartree–Fock (UHF ), unrestricted second-order Møller–Plesset (UMP 2) perturbation, unrestricted coupled cluster (UCCD ), and unrestricted quadratic configuration interaction (UQCISD ) calculations have been performed on the organic radicals CH₃, CH₃CH₂, CH₂CHCH₂, CH₃CHCOO^?, HCOCHCOH, CH₃COCHCOH, CH₃COCHCOCH₃, and CH₃COC(CH₃)COCH₃, using double-zeta and split-valence-plus-polarization basis sets. These radicals are derived from common organic ligands and have been observed in recent experimental work on tris(β-ketoenolato)cobalt(III) complexes. Their geometry has been optimized at the UHF level using the two mentioned basis sets. From these calcuations, values for the isotropic hyperfine coupling constants at the hydrogen atoms are predicted and compared with the experimental results. The usefulness of semiempirical extrapolations based on limited basis sets and treatment of electron correlation effects is carefully analyzed in the examples considered. © 1994 John Wiley & Sons, Inc.     

AB initio calculations of 2H and 14N quadrupolar coupling constants in hydrogen bonded dimers

SCF MO calculations at the 6-31G^** level of approximation are reported for ²H and ¹⁴N electric field gradients in HCN?HCN, HCN?HF, and CH₃CN?HF dimers, with emphasis on the configurational dependence of these quantities in (HCN)₂. In comparison with available experimental nuclear quadrupolar coupling constants, the calculated values for the monomers and dimers exhibit an accuracy of ≈ 10%, which is comparable to that of other spectroscopic parameters. The implications of hydrogen bonding for quadrupolar spin-lattice relaxation rates are briefly discussed.     

An indo investigation of magnetic resonance constants of azaaromatic systems

INDO–MO calculations have been performed on the ESR hyperfine coupling constants of the pyridinyl radical and the radical anions of pyrimidine, quinoline, isoquinoline, acridine and benzcinnoline. The nuclear spin coupling constants and the ¹⁴N nuclear quadrupole coupling constant of pyridine have also been calculated. In general, calculated values are in satisfactory agreement with the experimental data.     

Microwave spectra of [15n] and [13c] pyridines,quadrupole coupling constants,dipole moment and molecular structure of pyridine

The microwave spectra of two ring-substituted species of pyridine have been remeasured, and the spectra of the two remaining species investigated and assigned.¹⁴N quadrupole coupling splittings were analyzed and used to determine the quadrupole coupling constants, while the derived center frequencies were fitted to give improved values for the rotational constants.A complete substitution structure can now be presented.The dipole moment of pyridine was redetermined from Stark measurements in the spectrum of [¹⁵N]pyridine.     

Evaluation of Gaussian basis sets in ab initio

Ab initio wave functions for NH₃ have been calculated using the STONG program, where N is 2 to 6, and with Gaussian 70, employing the 4–31 and 6–31 basis sets. Electric field gradients at the N atom were computed from the gaussian basis orbitals, and, with STONG, also from the equivalent STO's. Results from the two split valence shell Gaussian 70 calculations are in good agreement with values from extended SCF and SCFCI calculations. Because of adequate agreement with more extended calculations, the use of the 4–31 basis set is chosen to calculate field gradients in molecules consisting of five or more second row atoms. It should be adequate for the prediction of the region for searching for new ¹⁴N, ¹⁷O and ²H NQR spectra. An improved value of the electric quadrupole moment for the ¹⁴N nucleus is proposed.     

Influence of Substituents in the Benzene Ring on the Halogen Bond of Iodobenzene with Ammonia

The effects on the C−I⋅⋅N halogen bond between iodobenzene and NH₃ of placing various substituents on the phenyl ring are monitored by quantum calculations. Substituents R=N(CH₃)₂, NH₂, CH₃, OCH₃, COCH₃, Cl, F, COH, CN, and NO₂ were each placed ortho, meta, and para to the I. The depth of the σ-hole on I is deepened as R becomes more electron-withdrawing which is reflected in a strengthening of the halogen bond, which varied between 3.3 and 5.5 kcal mol⁻¹. In most cases, the ortho placement yields the largest perturbation, followed by meta and then para, but this trend is not universal. Parallel to these substituent effects is a progressive lengthening of the covalent C−I bond. Formation of the halogen bond reduces the NMR chemical shielding of all three nuclei directly involved in the C−I⋅⋅N interaction. The deshielding of the electron donor N is most closely correlated with the strength of the bond, as is the coupling constant between I and N, so both have potential use as spectroscopic measures of halogen bond strength.     

Axial ligand influence on geometries, charge distributions and electronic structures of iron tetraazamacrocycle [FeTIM(X)(Y)] complexes assessed by Density Functional Theory

We employed the Density Functional Theory along with small basis sets, B3LYP/LANL2DZ, for the study of FeTIM complexes with different pairs of axial ligands (CO, H₂O, NH₃, imidazole and CH₃CN). These calculations did not result in relevant changes of molecular quantities as bond lengths, vibrational frequencies and electronic populations supporting any significant back-donation to the carbonyl or acetonitrile axial ligands. Moreover, a back-donation mechanism to the macrocycle cannot be used to explain the observed changes in molecular properties along these complexes with CO or CH₃CN. This work also indicates that complexes with CO show smaller binding energies and are less stable than complexes with CH₃CN. Further, the electronic band with the largest intensity in the visible region (or close to this region) is associated to the transition from an occupied 3d orbital on iron to an empty π^∗ orbital located at the macrocycle. The energy of this Metal-to-Ligand Charge Transfer (MLCT) transition shows a linear relation to the total charge of the macrocycle in these complexes as given by Mulliken or Natural Population Analysis (NPA) formalisms. Finally, the macrocycle total charge seems to be influenced by the field induced by the axial ligands.     

Adducts and dimers of SFn+ (n = 1–5) with benzene,acetonitrile, and pyridine. Gas-phase generation and ab initio structures and thermochemistry

Pentaquadrupole (QqQqQ) mass spectrometry is used to explore the abilities of gaseous SF_n⁺ (n = 1–5) ions to form adducts and dimers with three π-electron rich molecules—benzene, acetonitrile, and pyridine, whereas ab initio calculations estimate most feasible structures, bond dissociation energies (BDEs), and reaction enthalpies of the observed products. With benzene, SF⁺ reacts by net H-by-SF^+· replacement. As suggested by the calculations, this novel benzene reaction forms ionized benzenesulfenyl fluoride, C₆H₅–SF^+·, via a Wheland-type intermediate that spontaneously loses a H atom. SF₃⁺ forms a rare, loosely bonded π complex with benzene, [Bz ⋯ SF₃]⁺, which is stable toward both H and HF loss. No dimer, Bz₂SF₃⁺, is formed. According to calculations, an unsymmetrically bonded, π-coordinated Bz₂SF₃⁺ dimer exists, i.e. (Bz–SF₃ ⋯ Bz)⁺, but its formation from [Bz ⋯ SF₃]⁺ is endothermic; hence, thermodynamically unfavorable. With acetonitrile, SF₂^+·, SF₃⁺, and SF₅⁺ form both adducts and dimers. CH₃–C^·N–SF₂⁺ (a new distonic ion) and CH₃CN–SF₅⁺ are covalently bonded, but CH₃CN ⋯ SF₃⁺ is loosely bonded. The binding natures of the acetonitrile adducts are reflected in the dimers; [CH₃CN–SF₂ ⋯ NCCH₃]^+· and [CH₃CN–SF₅ ⋯ NCCH₃]⁺ are unsymmetrically bonded, whereas [CH₃CN ⋯ SF₃ ⋯ NCCH₃]⁺ is symmetrically and loosely bonded. Such dimers as [CH₃CN ⋯ SF₃ ⋯ NCCH₃]⁺ are ideal for measurements of ion affinity via the Cooks’ kinetic method. With pyridine, only SF₃⁺ forms adduct and dimer. Py–SF₃⁺ is covalently bonded through nitrogen; [Py ⋯ SF₃ ⋯ Py]⁺ is loosely but unsymmetrically bonded. The unsymmetric 2.28 and 2.44 Å long N–S bonds in [Py ⋯ SF₃ ⋯ Py]⁺, which are expected to rapidly interconvert, result likely from steric hindrance that forces orthogonal alignment of the two pyridine rings. Most observed adducts and dimers display relatively high BDEs, i.e. they are formed in thermodynamically favorable reactions. The extents of dissociation of the adducts and dimers observed in MS³ experiments reflect the structures and BDEs predicted by the calculations.     

Theoretical study of substituent effects on the acidity of the methyl group: Structure of anions CH2X−

Geometries have been optimized using molecular-orbital calculations (a) with a 4-31G Gaussian basis set for carbanions CH₂X^? where X = H, CH₃, NH₂, OH, F, C?CH, CH?CH₂, CHO, COCH₃, CN, and NO₂; and (b) with an STO -3G basis set for methyl acetate and acetyl deprotonated methyl acetate. All the carbanions containing unsaturated substituents are planar, with a considerable shortening of the C? X bond. Carbanions containing saturated substituents are pyramidal with the out-of-plane angle α increasing with the electronegativity of the substituent. Double-zeta basis set calculations give proton affinities over the range 449 (for CH₃CH₂^?) to 355 kcal/mol (for CH₂NO₂^?), with all unsaturated anions having smaller affinities than saturated anions. The correlation of proton affinities with 1s binding energies, and with charges on both the carbon of the anion and on the acidic proton of the neutral molecule are examined.     

Calculation of exchange coupling constants in solid state transition metal compounds using localized atomic orbital basis sets

The application of theoretical methods based on density functional theory using hybrid functionals and localized, atomic orbital type basis sets is shown to provide good estimates for exchange coupling constants in non-metallic, solid state transition metal compounds with relatively complex crystal structures. The accuracy of the calculated exchange coupling constants is similar to that previously obtained for dinuclear and polynuclear molecular compounds. As an application of this procedure, the magnetic properties of the high-temperature phase of CuGeO₃, the recently synthesized silver copper oxide Ag₂Cu₂O₃, and the family of M[N(CN)₂]₂ (M=Cr(II), Mn(II), Fe(II), Co(II), Ni(II) and Cu(II)) compounds are analyzed via the computation of their most relevant exchange coupling constants.     

Analysis of the ν2 :ν3 + ν4 Fermi resonance in CH3CN isotopic species in solution

Infrared bands due to ν₃ + ν₄, ν₂, ν₃ and ν₄ were recorded for the species CH₃CN, ¹³CH₃CN, CH₃¹³CN and CH₃C¹⁵N in the solvents carbon tetrachloride, chloroform, benzene, pyridine and dimethyl sulphoxide. Values of W₂₃₄ were extracted by the modified Winther method, which are slightly less, 11.2–11.9 cm⁻¹, than the value in the gas, 12.15 cm⁻¹. Relative intensity measurements of ν₃ + ν₄ and ν₂ in CCl₄ are compatible with an unperturbed intensity ratio, I_{ν3 + ν4}/ I_ν2 of ∼ 0.03. The fallibility of the infrared intensity method for determining W, and the need for precise frequency data in the Winther method, are stressed.