Validation of the M-C/H-C bond enthalpy relationship through application of density functional theory

Density functional theory has been used to calculate H-C and M-C bond dissociation enthalpies in order to evaluate the feasibility of correlating relative M-C bond enthalpies Delta H(M-C)rel with H-C bond enthalpies Delta H(H-C) via computational methods. This approach has been tested against two experimental correlations: a study of (a) Rh(H)(R)(Tp')(CNCH2CMe3) [R = hydrocarbyl, Tp' = HB(3,5-dimethylpyrazolyl)3] (Wick, D. D.; Jones, W. D. Organometallics 1999, 18, 495) and (b) Ti(R)(silox)2(NHSit-Bu3) (silox = OSit-Bu3) (Bennett, J. L.; Wolczanski, P. T. J. Am. Chem. Soc. 1997, 119, 10696). We show that the observation that M-C bond enthalpies increase more rapidly with different substituents than H-C bond enthalpies is reproduced by theory. Quantitative slopes of the correlation lines are reproduced within 4% of the experimental values with a B3PW91 functional and with very similar correlation coefficients. Absolute bond enthalpies are reproduced within 6% for H-C bonds, and relative bond enthalpies for M-C bonds are reproduced within 30 kJ mol(-1) for Rh-C bonds and within 19 kJ mol(-1) for Ti-C bonds. Values are also calculated with the BP86 functional.     

Microwave spectrum, structure, and quantum chemical studies of a compound of potential astrochemical and astrobiological interest: Z-3-amino-2-propenenitrile

Z-3-Amino-2-propenenitrile, H2NCH=CHCN, a compound of astrochemical and astrobiological interest, has been studied by Stark and Fourier transform microwave spectroscopy along with eight of its isotopologues; the synthesis of five of these are reported. The spectra of the ground vibrational state and of three vibrationally excited states belonging to the two lowest normal modes were assigned for the parent species, whereas the ground states were assigned for the isotopologues. The frequency of the lowest in-plane bending fundamental vibration was determined to be 152(20) cm(-1) and the frequency of the lowest out-of-plane fundamental mode was found to be 176(20) cm(-1) by relative intensity measurements. A delicate problem is whether this compound is planar or slightly nonplanar. It was found that the rotational constants of the nine species cannot be used to conclude definitely whether the molecule is planar or not. The experimental dipole moment is mu(a) = 16.45(12), mu(b) = 2.86(6), mu(c) = 0 (assumed), and mu(tot.) = 16.70(12) x 10(-30) C m [5.01(4) D]. The quadrupole coupling constants of the two nitrogen nuclei are chi(aa) = -1.4917(21) and chi(cc) = 1.5644(24) MHz for the nitrogen atom of the cyano group and chi(aa) = 1.7262(18) and chi(cc) = -4.0591(17) MHz for the nitrogen atom of the amino group. Extensive quantum-chemical calculations have been performed, and the results obtained from these calculations have been compared with the experimental values. The equilibrium structures of vinylamine, vinyl cyanide, and Z-3-amino-2-propenenitrile have been calculated. These calculations have established that the equilibrium structure of the title compound is definitely nonplanar. However, the MP2/VQZ energy difference between the planar and nonplanar forms is small, only -423 J/mol. Z-Amino-2-propenenitrile and E-3-amino-2-propenenitrile are formed simply by mixing ammonia and cyanoacetylene at room temperature. A plausible reaction path has been modeled. G3 calculations indicate that the enthalpy (298.15 K, 1 atm) of the transition state is about 130 kJ/mol higher than the sum of the enthalpies of the reactants ammonia and cyanoacetylene. This energy difference is comparatively high, which indicates that both E- and Z-3-aminopropenenitrile are not likely to be formed in the gas phase in cold interstellar clouds via a collision between ammonia and cyanoacetylene. An alternative reaction between protonated cyanoacetylene (H-C[triple bond]C-C[triple bond]NH+) and ammonia is predicted to have a much lower activation energy than the reaction between the neutral molecules. Although protonated E- and Z-3-aminopropenenitrile in principle may be formed this way, it is more likely that a collision between NH3 and H-C[triple bond]C-C[triple bond]NH+ leads to NH4+ and H-C[triple bond]C-C[triple bond]N.     

Binding of genistein to the estrogen receptor based on an experimental electron density study

In a continuing effort to determine a relationship between the biological function and the electronic properties of steroidal and nonsteroidal estrogens by analysis of the submolecular properties, an experimental charge density study has been pursued on the nonsteroidal phytoestrogen, genistein. X-ray diffraction data were obtained using a Rigaku R-Axis Rapid high-power rotating anode diffractometer with a curved image plate detector at 20(1) K. The total electron density was modeled using the Hansen-Coppens multipole model. Genistein packs in puckered sheets characterized by intra- and intermolecular hydrogen bonds while weaker intermolecular hydrogen bonds (O...H-C) exist between the sheets. A topological analysis of the electron density of genistein was then completed to characterize all covalent bonds, three O...H-O and four O...H-C intermolecular hydrogen bonds. Two O...H-O hydrogen bonds are incipient (partially covalent) type bonds, while the other O...H-O hydrogen bond and O...H-C hydrogen bonds are of the pure closed-shell interaction type. In addition, two intermolecular H...H interactions have also been characterized from the topology of the electron density. The binding of genistein to the estrogen receptor is discussed in terms of the electrostatic potential derived from the electron density distribution.     

Methyl formate and its mono and difluoro derivatives: conformational manifolds, basicity, and interaction with HF theoretical investigation

The conformational manifolds, scenarios of protonation, and hydrogen bond propensity of methyl formate and its mono and difluoro derivatives, which possess two oxygen atoms with different basicities, are studied at the B3LYP/6-311++G(3df,3pd) computational level. The optimized geometries of the title molecules, their energetics, and relevant harmonic vibrational frequencies, mainly of the ν(CH) mode of the H-C═O group, are of a primary focus. The Natural Bond Orbital analysis is invoked to obtain the second-order intra- or intermolecular hyperconjugation energies, occupations of antibonding orbitals, and hybridization of the carbon atoms. It is demonstrated that the Z conformers (and their rotamers) of the three title molecules are characterized by a higher stability compared to the E ones. The stabilities depend on the intramolecular hyperconjugative interaction and on the attraction or repulsion nonbonded interaction. The proton affinity of the carbonyl oxygen exceeds, by 15-20 kcal·mol(-1), that of the methoxy oxygen. Fluorine substitution causes a moderate lowering of the proton affinity of the oxygens. Protonation on the oxygen atoms yields a contraction of the C-H bond and large concomitant blue shift of the ν(CH) vibration. These changes are mainly determined by a lowering of the occupation of the corresponding σ*(CH) orbitals. The esters under consideration are probed on the interaction with the HF molecule. The complexes that are formed under this interaction on the oxygen of the H-C═O group are stronger than those formed on the oxygen belonging to the methoxy one. It is deduced that the hydrogen bond energies show a linear dependence on the proton affinities of the corresponding oxygen atoms. Hydrogen-bonded complexes of moderate strength are also formed, while HF interacts with the fluorine atoms of the fluorinated esters.     

Natural bond orbital analysis, electronic structure, non-linear properties and vibrational spectral analysis of L-histidinium bromide monohydrate: a density functional theory

The spectroscopic properties of the crystallized nonlinear optical molecule L-histidinium bromide monohydrate (abbreviated as L-HBr-mh) have been recorded and analyzed by FT-IR, FT-Raman and UV techniques. The equilibrium geometry, vibrational wavenumbers and the first order hyperpolarizability of the crystal were calculated with the help of density functional theory computations. The optimized geometric bond lengths and bond angles obtained by using DFT (B3LYP/6-311++G(d,p)) show good agreement with the experimental data. The complete assignments of fundamental vibrations were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. The natural bond orbital (NBO) analysis confirms the occurrence of strong intra and intermolecular N-H?O hydrogen bonding.     

The conversion of phenylpropanedinitrile (phenylmalononitrile) into the carbanion, followed by IR spectra, ab initio and DFT force field calculations

The spectral and structural changes, caused by the conversion of phenylpropanedinitrile (phenylmalononitrile) into the carbanion, have been followed by IR spectra, ab initio HF, MP2 and DFT BLYP force field calculations. In agreement between theory and experiment, the conversion is accompanied with strong frequency decreases (with 114 cm(-1), mean value) of the cyano stretching bands nu(C triple bond N), dramatic increases in the corresponding integrated intensities (136-fold, total value), strong enhancement of the nu(C triple bond N) vibrational coupling and other essential spectral changes. According to the calculations, the strongest structural changes take place at the carbanionic center: (i) shortenings of the Cz-Ph and Cz-CN bonds with 0.064-0.092 A, and increases in the corresponding bond orders with 0.14-0.21 U; (ii) simultaneous enlargements of the bond angles at the same carbon atom with 7.6 degrees -9.7 degrees, as from tetrahedral its configuration becomes trigonal. The carbanionic charge is distributed between the two cyano groups (0.44-0.52 e(-)), phenyl ring (0.31-0.34 e(-)) and carbanionic center (0.14-0.25 e(-)). The formation of moderately strong (CH(3))(2)S=O...H-C(CN)(2)C(6)H(5) hydrogen bonds has been found experimentally.     

Ab initio vibration and ro-vibrational spectrum of the 1A1 state of He2Si2+

The low-lying ro-vibrational states for the ground electronic state (1A1) of HeSi2+ have been calculated using an ab initio variational solution of the nuclear Schr?dinger equation. A 96 point CCSD(T)/cc-pCVQZ potential energy surface (PES) has been calculated and a Ogilvie-Padé (3,6) potential energy function has been generated. This force field was embedded in the Eckart-Watson Hamiltonian from which the vibrational and ro-vibrational eigenfunctions and eigenenergies have been variationally calculated. A 70 point QCISD/aug-cc-pCVTZ discrete dipole moment surface (DMS) was calculated and a 5th order power series expansion (in terms of the two bond lengths and the included bond angle) has been generated. Absolute line intensities have been calculated and are presented for some of the most intense transitions between the vibrational ground state and the low-lying ro-vibrational states of this ion.     

Quantum chemistry aspects of the solvent effects on the ene reaction of 1‐Phenyl‐1,3,4‐triazolin‐2,5‐dione and 2‐methyl‐2‐butene

A density functional theory study was used to investigate the quantum aspects of the solvent effects on the kinetic and mechanism of the ene reaction of 1‐phenyl‐1,3,4‐triazolin‐2,5‐dione and 2‐methyl‐2‐butene. Using the B3LYP/6–311++ G(d,p) level of the theory, reaction rates have been calculated in the various solvents and good agreement with the experimental data has been obtained. Natural bond orbital analysis has been applied to calculate the stabilization energy of N18? H19 bond during the reaction. Topological analysis of quantum theory of atom in molecule (QTAIM) studies for the electron charge density in the bond critical point (BCP) of N18? H19 bond of the transition states (TSs) in different solvents shows a linear correlation with the interaction energy. It is also seen form the QTAIM analysis that increase in the electron density in the BCP of N18? H19, raises the corresponding vibrational frequency. Average calculated ratio of 0.37 for kinetic energy density to local potential energy density at the BCPs as functions of N18? H19 bond length in different media confirmed covalent nature of this bond. Using the concepts of the global electrophilicity index, chemical hardness and electronic chemical potentials, some correlations with the rate constants and interaction energy have been established. Mechanism and kinetic studies on 1‐phenyl‐1,3,4‐triazolin‐2,5‐dione and 2‐methyl‐2‐butene ene reaction suggests that the reaction rate will boost with interaction energy enhancement. Interaction energy of the TS depends on the solvent nature and is directly related to electron density of the bonds involved in the reaction proceeding, global electrophilicity index and electronic chemical potential. However, the chemical hardness relationship is reversed. Finally, an interesting and direct correlation between the imaginary vibrational frequency of the N18? H19 critical bond and its electron density at the TS has been obtained. © 2014 Wiley Periodicals, Inc.     

Ab initio/DFT electronic structure calculations,spectroscopic studies of 5-bromo-2-pyridinecarbonitrile – A comparative study

Fourier transform infrared (FTIR) and FT-Raman spectra have been recorded and extensive spectroscopic investigations have been carried out on 5-bromo-2-pyridinecarbonitrile (5B2PC). The optimized geometries, wavenumber and intensity of the vibrational bands of (5B2PC) have been calculated using density functional level of theory (DFT/B3LYP) employing 6-311G(d,p) basis set. On the basis of the comparison between calculated and experimental results, assignments of the fundamental vibrational modes are examined. The molecular stability and bond strength were investigated by applying the natural bond orbital (NBO) analysis. The electronic properties like HOMO–LUMO analysis of (5B2PC) have been reported.     

Triangular Halogen Bond and Hydrogen Bond Supramolecular Complex Consisting of Carbon Tetrabromide,Halide, and Solvent Molecule： A Theoretical and Spectroscopic Study

The theoretical calculation and spectroscopic experiments indicate a kind of triangular three bonding supramolecular complexes CBr4…X^-…-H-C, which consist of carbon tetrabromide, halide, and protic solvent molecule （referring to dichloromethane, chloroform and acetonitrile）, can be formed in solution. The strength of halogen and hydrogen bonds in the triangular complexes using halide as common acceptor obeys the order of iodide〉bromide〉chloride. The halogen and hydrogen bonds work weak-cooperatively. Charge transfer bands of halogen bonding complexes between CBra and halide are observed in UV-Vis absorption spectroscopy in three solvents, and then the stoichiometry of 1：1, formation constants K and molar extinction coefficients ε of the halogen bonding complexes are obtained by Benesi-Hildebrand method. The K and ε show a dependence on the solvent dielectric constant and, on the whole, obey an order of iodide〉bromide〉chloride in the same solvents. Furthermore, the C-H vibrational frequencies of solvent molecules vary obviously with the addition of halide, which indicates the C-H…X- interaction. The experimental data indicate that the halogen bond and hydrogen bond coexist by sharing a common halide acceptor as predicted by calculation.     

乙烯、乙炔和苯与氨基锂间的π型锂键相互作用

分别在DFT-B3LYP和MP2/6-311++G**水平上求得乙烯、乙炔和苯与氨基锂锂键复合物势能面上的3个几何构型. 频率分析表明,3个构型均为稳定构型. 计算结果表明,形成锂键复合物后,质子供体N(2)- Li(4)的键长明显增大,且其伸缩振动的频率发生了不同程度的红移. 分别在乙烯…氨基锂、乙炔…氨基锂和苯…氨基锂三种复合物中,经MP2/6-311++G**水平计算的同时含基组重叠误差(BSSE)和零点振动能校正的单体间锂键相互作用能分别为-26.04、-24.86 和 -30.02 kJ·mol-1. 自然键轨道理论（NBO）分析表明单体间的弱相互作用属于π-s型锂键.     

NMR spectroscopic data of some 1-alkoxy-2,2-di(carbonyl, carboxyl, cyano)-substituted ethylenes

The 1H-13C NMR shifts as well as 1H and 13C coupling constants of 14 alkoxymethylene malonic acid and acetoacetic acid derivatives and two alkoxymethylene acetylacetones are reported. The 17O NMR spectra have been recorded for six of them. The long-range coupling 3J(H-C=C-C(R)) has been used for determining the stereochemistry of the double bond.     

Bond energy M-C/H-C correlations: dual theoretical and experimental approach to the sensitivity of M-C bond strength to substituents

DFT methods are used to quantify the relationship between M-C and H-C bond energies; for MLn = Re(eta5-C5H5)(CO)2H and fluorinated aryl ligands, theoretical and experimental investigations of ortho-fluorine substitution indicate a much larger increase in the M-C than in the H-C bond energy, so stabilising C-H activation products.     

HC[triple bond]P and H3C-C[triple bond]P as proton acceptors in protonated complexes containing two phosphorus bases: structures, binding energies, and spin-spin coupling constants

Ab initio calculations at the MP2/aug'-cc-pVTZ level have been carried out to investigate the structures and binding energies of cationic complexes involving protonated sp, sp2, and sp3 phosphorus bases as proton donor ions and the sp-hybridized phosphorus bases H-C[triple bond]P and H3C-C[triple bond]P as proton acceptors. These proton-bound complexes exhibit a variety of structural motifs, but all are stabilized by interactions that occur through the pi cloud of the acceptor base. The binding energies of these complexes range from 6 to 15 kcal/mol. Corresponding complexes with H3C-C[triple bond]P as the proton acceptor are more stable than those with H-C[triple bond]P as the acceptor, a reflection of the greater basicity of H3C-C[triple bond]P. In most complexes with sp2- or sp3-hybridized P-H donor ions, the P-H bond lengthens and the P-H stretching frequency is red-shifted relative to the corresponding monomers. Complex formation also leads to a lengthening of the C[triple bond]P bond and a red shift of the C[triple bond]P stretching vibration. The two-bond coupling constants 2pihJ(P-P) and 2pihJ(P-C) are significantly smaller than 2hJ(P-P) and 2hJ(P-C) for complexes in which hydrogen bonding occurs through lone pairs of electrons on P or C. This reflects the absence of significant s electron density in the hydrogen-bonding regions of these pi complexes.     

Hydrogen bonding interaction and topological insights of the electron localization/delocalization of l- arginine acetate

The vibrational spectral studies of the semi-organic material l- arginine acetate (LAA) are carried out with the help of density functional calculations to derive the equilibrium geometry as well as the vibrational wavenumbers and intensities of the spectral bands. The vibrational spectrum assignments are performed using normal coordinate analysis (NCA) in accordance with the scaled quantum mechanical force field approach (SQMFF). Vibrational spectra confirm the COO- modes split due to intra- and intermolecular association based on C–O….H, N–H….O, and O–H?O hydrogen bonding in the molecule, which lowers carboxylate wavenumbers. The natural bond orbital (NBO) analysis and DFT computations also confirm the occurrence of strong intra and intermolecular N–H?O and O–H?O ionic hydrogen bonding between charged species, providing the non-centrosymmetric structure in the LAA crystal.     

Time‐dependent density functional theory study on the electronic excited‐state hydrogen‐bonding dynamics of 4‐aminophthalimide (4AP) in aqueous solution: 4AP and 4AP–(H2O)1,2 clusters

The time‐dependent density functional theory (TDDFT) method has been carried out to investigate the excited‐state hydrogen‐bonding dynamics of 4‐aminophthalimide (4AP) in hydrogen‐donating water solvent. The infrared spectra of the hydrogen‐bonded solute?solvent complexes in electronically excited state have been calculated using the TDDFT method. We have demonstrated that the intermolecular hydrogen bond C? O···H? O and N? H···O? H in the hydrogen‐bonded 4AP?(H₂O)₂ trimer are significantly strengthened in the electronically excited state by theoretically monitoring the changes of the bond lengths of hydrogen bonds and hydrogen‐bonding groups in different electronic states. The hydrogen bonds strengthening in the electronically excited state are confirmed because the calculated stretching vibrational modes of the hydrogen bonding C?O, amino N? H, and H? O groups are markedly red‐shifted upon photoexcitation. The calculated results are consistent with the mechanism of the hydrogen bond strengthening in the electronically excited state, while contrast with mechanism of hydrogen bond cleavage. Furthermore, we believe that the transient hydrogen bond strengthening behavior in electroniclly excited state of chromophores in hydrogen‐donating solvents exists in many other systems in solution. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

HF and DFT studies of the structure and vibrational spectra of 8-hydroxyquinoline and its mercury(II) halide complexes

The geometry, frequency and intensity of the vibrational bands of 8-hydroxyquinoline (8-HOQ) were obtained by HF and density functional theory (DFT) with BLYP and B3LYP functionals and 6-31G(d) as the basis set. The optimized bond lengths and bond angles are in good agreement with the X-ray data. The vibrational spectra of 8-HOQ which is calculated by the HF and DFT methods, reproduces the vibrational wavenumbers and intensities with an accuracy, which allows reliable vibrational assignments. Complexes of the type Hg(8-HOQ)X(2) [where X = Cl , Br] have been studied in the 4000-200 cm(-1) region, and assignment of all the observed bands were made. The analysis of the infrared spectra indicates that there are some structure-spectra correlations.     

Density Functional Theory Study of the Interaction between Thymine and Luteolin

The density function B3LYP method has been used to optimize the geometries of the luteolin, thymine and luteolin‐thymine complexes at 6‐31+G?? basis. The vibrational frequencies have been studied at the same level to analyze these seventeen complexes, respectively. Theories of atoms in molecules (AIM) and natural bond orbital (NBO) have been utilized to investigate the hydrogen bonds involved in all the systems. The interaction energies of the complexes corrected by basis set superposition error are between ?93.00–?76.69 kJ/mol. The calculating results indicate that strong hydrogen bonding interactions have been found in the luteolin‐thymine complexes.     

Inversion vibration of PH3+(X2A2") studied by zero kinetic energy photoelectron spectroscopy

We report the first rotationally resolved spectroscopic studies on PH3+(X2A2") using zero kinetic energy photoelectron spectroscopy and coherent VUV radiation. The spectra about 8000 cm(-1) above the ground vibrational state of PH3+(X2A2") have been recorded. We observed the vibrational energy level splittings of PH3+(X2A2") due to the tunneling effect in the inversion (symmetric bending) vibration (nu2+). The energy splitting for the first inversion vibrational state (0+/0-) is 5.8 cm(-1). The inversion vibrational energy levels, rotational constants, and adiabatic ionization energies (IEs) for nu2+ = 0-16 have been determined. The bond angles between the neighboring P-H bonds and the P-H bond lengths are also obtained using the experimentally determined rotational constants. With the increasing of the inversion vibrational excitations (nu2+), the bond lengths (P-H) increase a little and the bond angles (H-P-H) decrease a lot. The inversion vibrational energy levels have also been calculated by using one dimensional potential model and the results are in good agreement with the experimental data for the first several vibrational levels. In addition to inversion vibration, we also observed firstly the other two vibrational modes: the symmetric P-H stretching vibration (nu1+) and the degenerate bending vibration (nu4+). The fundamental frequencies for nu1+ and nu4+ are 2461.6 (+/-2) and 1043.9 (+/-2) cm(-1), respectively. The first IE for PH3 was determined as 79670.9 (+/-1) cm(-1).     

Computed vibrational frequencies of actinide oxides AnO(0/+/2+) and AnO2(0/+/2+) (An = Th, Pa, U, Np, Pu, Am, Cm)

The vibrational frequencies of the actinide oxides AnO and AnO(2) (An = Th, Pa, U, Np, Pu, Am, Cm) and of their mono- and dications have been calculated using advanced quantum chemical techniques. The stretching fundamental frequencies of the monoxides have been determined by fitting the potential function to single-point energies obtained by relativistic CASPT2 calculations along the stretching coordinate and on this basis solving numerically the ro-vibrational Schro?dinger equation. To obtain reliable fundamental frequencies of the dioxides, we developed an empirical approach. In this approach the harmonic vibrational frequencies of the AnO(2)(0/+/2+) species were calculated using eight different exchange-correlation DFT functionals. On the basis of the good correlation found between the vibrational frequencies and computed bond distances, the final frequency values were derived for the CASPT2 reference bond distances from linear regression equations fitted to the DFT data of each species. As a test, the approach provided excellent agreement with accurate experimental data of ThO, ThO(+), UO, and UO(+). The joint analysis of literature experimental and our computed data facilitated the prediction of reliable gas-phase molecular properties for some oxides. They include the stretching frequencies of PuO, ThO(2), UO(2), and UO(2)(+) and the bond distance of PuO (1.818 ?, being likely within 0.002 ? of the real value). Also the derived equilibrium bond distances of ThO(2), UO(2), and UO(2)(+) (1.896, 1.790, and 1.758 ?, respectively) should approximate closely the (yet unknown) experimental values. On the basis of the present results, we suggest that the ground electronic state of PuO(2) in Ar and Kr matrices is probably different from that in the gaseous phase, similarly to UO(2) observed previously.