Internal Rotation and Equilibrium Structure of the Bromonitromethane Molecule According to Gas Electron Diffraction Data and Quantum Chemical Calculations

The structure and internal rotation of the bromonitromethane molecule are studied using electron diffraction analysis and quantum chemical calculations. The electron diffraction data are analyzed within the models of a general intramolecular anharmonic force field and quantum chemical pseudoconformers to account for the adiabatic separation of a large amplitude motion associated with the internal rotation of the NO₂ group. The following experimental bond lengths and valence angles are obtained for the equilibrium orthogonal configuration of the molecule with C_s symmetry: r_e(N=O) = 1.217(5) Å, r_e(C–N) = 1.48(2) Å, r_e(C–Br) = 1.919(5) Å, ∠_еBr–C–N = 109.6(9)°, ∠_еO=N=O = 125.9(9)°. The equilibrium geometry parameters are in good agreement with CCSD(T)/cc-pVTZ calculations. Thermally averaged parameters are calculated using the equilibrium geometry and quadratic and cubic quantum chemical force constants. The barrier to internal rotation cannot be determined reliably based on the electron diffraction data used in this work. There is a 82% probability that the equilibrium configuration with orthogonal C–Br and N=O bonds is most preferable, and internal rotation barrier does not exceed 280 cm^-1, which agrees with CCSD(T)/cc-pVTZ calculations.     

Spectroscopic properties of HCNH+ calculated by SCEP CEPA

Spectroscopic properties for various isotopomers of HCNH⁺ were calculated by SCEP CEPA using a basis set of 80 contracted GTOs. The equilibrium bond lengths are predicted to be r_1e(CH) = 1.0785, R_e(CN) = 1.1346, and r_2e(NH) = 1.0116 Å. Anharmonic stretching frequencies and IR intensities were calculated. The strongest stretching band is ν₂(H¹²C¹⁴ND⁺ ) at 2681 cm⁻¹ with an absorption band strength of 1676 atm⁻¹ cm⁻² at 298 K. The equilibrium dipole moment of H¹²C¹⁴NH⁺ is −0.29±0.02 D.     

Quantum-chemical calculations for silyl- and alkyl-pseudohalides X3YNCZ (X = H or CH3; Y = C or Si; Z = O,S, or Se) and electron diffraction study of the equilibrium molecular structure of (CH3)3SiNCSe

The equilibrium structures and force fields of the twelve simplest silyl- and alkyl-pseudo halides are calculated by means of B3LYP and MP2(full) quantum-chemical methods with the use of the aug-cc-pVTZ basis. Some regularities in their structure are established. Using these data, the equilibrium structure of the (CH₃)₃SiNCSe molecule with symmetry C _3v is described experimentally for the first time via gas electron diffraction. The following values of the main r _e parameters are determined (uncertainty 3σ is in parentheses): C=Se, 1.709(14) Å; N=C, 1.190(10) Å; N-Si, 1.767(15) Å; Si-C, 1.847(13) Å; N-Si-C, 106.4°; C-Si-C, 112.4°.     

Experimental and computed bond lengths: The importance of their differences

State-of-the-art experimental electron diffraction and computational information on the structure of alkaline earth dihalide molecules are in agreement for the shape of these symmetric triatomic molecules (linear/bent/quasi-linear). However, the computed and measured bond lengths show differences that are not only considerably larger than the experimental error but also have the wrong sign. The physical meaning of experimental bond lengths depends on the physical techniques used in their determination and the ways of averaging over molecular vibrations. The choice of model potentials in the elucidation of experimental information is also important, especially for floppy molecules. When improved computational bond lengths become available, their comparison with experimental information will have to take account of the physical meaning of the experimentally determined bond lengths. The computed equilibrium distance (r_e) should be smaller than the experimental distance-average bond length (r_g). The differences may range from a few thousandths of an Å to a few hundredths with increasing temperature and, especially, with increasing floppiness of the molecule. For truly accurate comparison, experimental bond lengths should be compared with computed ones only following necessary corrections, bringing all information involved in the comparison to a common denominator. © 1992 John Wiley & Sons, Inc.     

On electronic states and bond lengths of the truncated icosahedral C60 molecule

The electronic states and the bond lengths of the truncated icosahedral C₆₀ molecule have been calculated by the Hückel and Coulson-Golebiewski self-consistent Hückel methods. C₆₀ has a stable closed shell with a rather big energy gap (= 0.847β) between the HOMO and the LUMO. We have obtained two kinds of bond lengths r₁ = 1.434 Å and r₂ = 1.403 Å, which correspond to the edges of the regular pentagon and the edge of a hexagon not lying on a pentagon.     

The equilibrium structures of linear carbon clusters of type C2 n +1 (n = 1–4)

On the basis of large-scale coupled-cluster calculations including connected triple substitutions accurate equilibrium structures have been established for the linear carbon chain molecules with an odd number of carbon atoms up to C₉. The individual equilibrium bond lengths are indicative of strong double bonds in all cases. Received: 16 November 1999 / Accepted: 5 December 1999 / Published online: 19 April 2000     

Calculation of bond dissociation energies of diatomic molecules using bond function basis sets with counterpoise corrections

Bond function basis sets combined with the counterpoise procedure are used to calculate the molecular dissociation energies D_e of 24 diatomic molecules and ions. The calculated values of D_e are compared to those without bond functions and/or counterpoise corrections. The equilibrium bond lengths r_e and harmonic frequencies o_e are also calculated for a few selected molecules. The calculations at the fourth-order Møller-Plesset approximation (MP 4) have consistently recovered about 95–99% of the experimental values for D_e; compared to as low as 75% without use of bond functions. The calculated values of r_e are typically 0.01 Å larger than the experimental values, and the calculated values of o_e are over 95% of the experimental values. © 1996 John Wiley & Sons, Inc.     

The crystal and molecular structures of bis (pentafluorophenyl)disulphide and bis(pentafluorophenyl)diselenide

The structures of (C₆F₅)₂S₂ and (C₆F₅)₂Se₂ have been determined by single crystal, X-ray diffraction techniques. The compounds are isostructural although the molecules are packed differently in the crystal in comparison with their phenyl analogues. Important bond lengths and angles are: SS, 2.059(4)Å; SeSe, 2.319(4)Å; SC, 1.770Å; SeC, 1.910(15)Å; SSC, 101.3(3)°; SeSeC, 98.8(1)°.     

The Scent of Maibowle – π Electron Localization in Coumarin from Its Microwave-Determined Structure

The microwave spectra of the natural substance coumarin, a planar aromatic molecule with the specific scent of maibowle, a popular fruit punch served in spring and early summer, were recorded using a molecular jet Fourier transform microwave spectrometer working in the frequency range from 4.0 to 26.5 GHz. The rotational constants and centrifugal distortion constants were determined with high precision, reproducing the spectra to experimental accuracy. The spectra of all singly-substituted ¹³C and ¹⁸O isotopologues were observed in their natural abundances to determine the experimental heavy atom substitution r_s and semi-experimental equilibrium r_e^SE structures. The experimental bond lengths and bond angles were compared to those obtained from quantum chemical calculations and those of related molecules reported in the literature with benzene as the prototype. The alternation of the C−C bond lengths to the value of 1.39 Å found for benzene reflects the localization of π electrons in coumarin, where the benzene ring and the lactone-like chain −CH=CH−(C=O)−O− are fused. The large, negative inertial defect of coumarin is consistent with out-of-plane vibrations of the fused rings.     

Toward spectroscopic accuracy of ab initio calculations of vibrational frequencies and related quantities: a case study of the HF molecule

Calculations at various coupled-cluster (CC) levels with and without the inclusion of linear r _{i j} -dependent terms are performed for the HF molecule in its ground state with a systematic variation of basis sets. The main emphasis is on spectroscopic properties such as the equilibrium distance r _e and the harmonic vibration frequency ω_e. Especially with the R12 methods (including linear r _{i j} -dependent terms), convergence to the basis set limit is reached. However, the results (at the basis set limit) are rather sensitive to the level of the treatment of electron correlation. The best results are found for the CCSDT1-R12 and CCSD[T]-R12 methods (CCSD[T] was previously called CCSD+T(CCSD)), while CCSD(T) overestimates ω_e by ≈6 cm⁻¹. The good agreement of conventional CCSD(T) with experiment for basis sets far from saturation (e.g. truncated at g-functions) is probably the result of a compensation of errors. The contribution of core-correlation is non-negligible and must be included (effect on ω_e≈5 cm⁻¹). Relativistic effects are also important (23 cm⁻¹), while adiabatic effects are much smaller (<1cm⁻¹) and non-adiabatic effects on ω_e can be simulated in replacing nuclear by atomic masses; for rotation nuclear masses appear to be the better choice, at least for hydrides. From a potential curve based on calculations with the CCSDT1-R12 method with relativistic corrections, the IR spectrum is computed quantum-mechanically. Both the band heads and the rotational structures of the observed spectra are reproduced with a relative error of ≈10⁻⁴ for the three isotopomers HF, DF, and TF. Received: 3 July 1998 / Accepted: 4 August 1998 / Published online: 28 October 1998     

Formation of Short Zn−Zn Bonds Stabilized by Simple Cyanide and Isocyanide Ligands

Cyanogen diluted in argon was reacted with laser ablated Zn atoms to produce the NCZnCN and NCZnZnCN cyanides and higher energy isocyanides ZnNC, CNZnNC, and CNZnZnNC, which were isolated in excess argon at 4 K. These reaction products, identified from the matrix infrared spectra of their ‐CN and ‐NC chromophore ligand stretching modes, were confirmed by ¹³C and ¹⁵N isotopic substitution and comparison with frequencies calculated by the B3LYP and CCSD(T) methods using the all electron aug‐cc‐pVTZ basis sets. The cyanide and isocyanide products were increased markedly by mercury arc UV photolysis, which covers the zinc atomic absorption. The above electronic structure calculations that produce appropriate ligand frequencies for these dizinc products also provide their Zn?Zn bond lengths: CCSD(T) calculations find a short 2.367 Å Zn?Zn bond in the NCZnZnCN cyanide, a shorter 2.347 Å Zn?Zn bond in the 37.4 kJ mol^?1 higher energy isocyanide CNZnZnNC, and a longer 4.024 Å bond in the dizinc van der Waals dimer. Thus, the diatomic cyanide (‐CN) and isocyanide (‐NC) ligands are as capable of stabilizing the Zn?Zn bond as many much larger ligands based on their measured and our calculated Zn?Zn bond lengths. This is the first example of dizinc complexes stabilized by different ligand isomers. Additional weaker bands in this region can be assigned to the analogous trizinc molecules NCZnZnZnCN and CNZnZnZnNC.     

Internal rotation and equilibrium structure of the 2-methyl-2-nitropropane molecule from joint processing of gas phase electron diffraction data,vibrational and microwave spectroscopy data,and quantum chemical calculation results

The structure and internal rotation of the 2-methyl-2-nitropropane molecule is studied by electron diffraction and quantum chemical calculations with the use of microwave and vibrational spectroscopy data. The electron diffraction data are analyzed within the general intramolecular anharmonic force field model and the quantum chemical pseudoconformer model, considering the adiabatic separation of the degree of freedom of large amplitude motion, i.e., the internal rotation of the NO₂ group. The equilibrium eclipsed configuration of the C _s symmetry molecule has the following experimental bond lengths and valence angles: r _e(N=O) = 1.226//1.226(8) Å, r _e(C–N)//r _e(C–C) = 1.520//1.515/1,521(4) Å, ∠_еC–C–N = = 109.1/106,1(8)°, ∠_еO=N=O = 124.2(6)°, ∠_eC–C–H_avg = 110(3)°. The equilibrium geometry parameters are well consistent with MP2/cc-pVTZ quantum chemical calculations and microwave spectroscopy data. The thermally average parameters previously obtained within the small vibration model show a satisfactory agreement with the new results. The electron diffraction data used in this work do not allow a reliable determination of the barrier to internal rotation. However, at a barrier of 203(2) cal/mol, which is derived from the microwave study, it follows from the electron diffraction data that the equilibrium configuration must correspond to an eclipsed arrangement of C–C and N=O bonds, which is also consistent with the results of quantum chemical calculations of various levels.     

Crystal and molecular structure of bis(N,N-dipropyl-N′-diphenoxythiophosphoryl-thioureato) nickel(II) [(C6H5O)2P(S)NC(S)N(C3H7)2]2Ni

The Ni complex [C₆H₅O₂P(S)N(C₃H₇₂]₂Ni is monoclinic, space group P2₁/n with a = 8.890(3), b = 21.692(5), c = 11.670(4) Å, β = 108.35(5)°, V = 2136(1) ų, F(000) = 916, M_r = 534.01, Z = 2, D_m = 1.318, D_x = 1.358 Mg m^?3, graphite monochromatized MoKα ^? radiation, π = 0.7107 Å, μ = 0.76 mm^?1, T = 293 K. The structure was solved by a heavy atom method and refined to R = 0.044 for 3095 independent reflexions. The Ni atom lies in the centre of symmetry and is coordinated by four S atoms of the two molecules of the ligand in a planar arrangement. Ni? S bond lengths are 2.205 and 2.226 Å resp., the angles S? Ni? S are 97.65 and 82.35° resp.     

On the geometrical structure and UV spectrum of the truncated icosahedral C60, molecule

The PPP CI molecular-orbital theory for three-dimensional systems has been applied to study the UV spectrum of the truncated icosahedral C₆₀ molecule. We have found that only the one-electron transitions to T_1u symmetry (4.2270,4.7498 and 6.5182 eV) have oscillator strengths different from zero. Using a bond-order-bond-length relation in SCF iteration connected to the PPP method, we have obtained two kinds of bond lengths r₁ = 1.439 Å and r₂ = 1.398 Å, which correspond to the edges of the regular pentagon and the edge of a hexagon not lying on a pentagon.     

Formation of Short Zn−Zn Bonds Stabilized by Simple Cyanide and Isocyanide Ligands

Cyanogen diluted in argon was reacted with laser ablated Zn atoms to produce the NCZnCN and NCZnZnCN cyanides and higher energy isocyanides ZnNC, CNZnNC, and CNZnZnNC, which were isolated in excess argon at 4 K. These reaction products, identified from the matrix infrared spectra of their -CN and -NC chromophore ligand stretching modes, were confirmed by ¹³C and ¹⁵N isotopic substitution and comparison with frequencies calculated by the B3LYP and CCSD(T) methods using the all electron aug-cc-pVTZ basis sets. The cyanide and isocyanide products were increased markedly by mercury arc UV photolysis, which covers the zinc atomic absorption. The above electronic structure calculations that produce appropriate ligand frequencies for these dizinc products also provide their Zn−Zn bond lengths: CCSD(T) calculations find a short 2.367 Å Zn−Zn bond in the NCZnZnCN cyanide, a shorter 2.347 Å Zn−Zn bond in the 37.4 kJ mol⁻¹ higher energy isocyanide CNZnZnNC, and a longer 4.024 Å bond in the dizinc van der Waals dimer. Thus, the diatomic cyanide (-CN) and isocyanide (-NC) ligands are as capable of stabilizing the Zn−Zn bond as many much larger ligands based on their measured and our calculated Zn−Zn bond lengths. This is the first example of dizinc complexes stabilized by different ligand isomers. Additional weaker bands in this region can be assigned to the analogous trizinc molecules NCZnZnZnCN and CNZnZnZnNC.     

On the structure and conformational composition of 1,1-bis(methylthio)ethylene

1,1-bis(methylthio)ethylene has been studied in the gaseous phase by electron diffraction and in the solid and liquid phases by Raman spectroscopy. While there is apparently only one conformer in the solid, the fluid phases consist of probably three forms, two of these have a non-planar skeleton. Average values for the bond lengths are: r_a(C_eth—S) = 1.767 Å, r_a(C_met—S) = 1.815 Å, r_a(CC) = 1.348 Å.     

Molecular geometries and heats of formation of C60 and C70 as computed by MM2-87

MM2-87 calculations have been performed on C₆₀ (buckminsterfullerence; footballene) and C₇₀ with full energy minimization. The steric energies for C₆₀ and C₇₀ were computed to be 179.9 and 177.9 kcal/mol, respectively. The heats of formation for C₆₀ is found to be more stable than C₆₀. The two bond lengths for C₆₀ were computed to be 1.393 and 1.444 Å. Eight different bond lengths were found for C₇₀ ranging from 1.386 to 1.452 Å. Bond angles, dihedral angles, and moments of inertia are also reported for the compounds.     

Vibrational force fields and amplitudes,and zero-point average structures of (CH3)3Y molecules (Y = N,P, As,Sb, Bi): A Combination of electron-diffraction and spectroscopic data

Following the development of methods for placing electron-diffraction and spectroscopic geometrical parameters on a common basis, available data on (CH₃)₃Y molecules (Y = N, P, As, Sb, Bi) have been used to derive force fields, r.m.s. amplitudes of vibration u along the internuclear vectors, and perpendicular amplitude correction coefficients K for these molecules. For trimethylamine, the amplitudes are similar whether or not off-diagonal elements are included in the force field; hence, only diagonal elements are considered for the other molecules. Among the interesting trends, as group V is descended, is that the C-Y r.m.s. amplitudes increase only from 0.049 to 0.058 Å, whereas the C-Y stretching force constant decreases by over 60% from 5.3 to 1.8 mdyn Å^?1. There is evidence for an increasing tendency for torsional motion of methyl groups, as group V is descended.For each of the molecules, the amplitude data were used to derive zero-point average (r^o_α) structures and to make estimates of a partial equilibrium (r_e) structure. For trimethylamine the results suggest a systematic error in the electron-beam wavelength of the literature study, and the structural parameters were appropriately revised. The r^o_α lengths of the C-Y bonds in the five molecules are 1.458 ± 0.002, 1.844 ± 0.003, 1.979 ± 0.010, 2.169 ± 0.010 and 2.263 ± 0.004 Å, respectively. The estimated r_e parameters for the bonds in trimethylamine agree well with the microwave r_s structure.     

Ab Initio rovibrational spectrum of the NaH2 + ion–quadrupole complex

The electronic and rovibrational structure of (¹A₁) NaH₂ ⁺ has been investigated using a relativistically-corrected, all-electron coupled-cluster with singles, doubles and perturbative triples (CCSD(T)) ansatz. For the electronic ground state this ansatz yielded equilibrium Na–H bond lengths (R _e ) of 2.4208 Å and an equilibrium H–Na–H bond angle (θ_e) of 17.8°. An analytical potential energy surface (PES) was embedded in the rovibrational Hamiltonian. The PES was constructed using 118 CCSD(T) points and exhibited a residual error of 1.2 cm^?1. The rovibrational Hamiltonian was diagonalised using variational techniques. The vibrational and rovibrational eigenvectors were assigned using a configuration weight scheme in terms of normal modes and the Mulliken assignment scheme, respectively. For the ground vibrational state of (¹A₁) NaH₂ ⁺, the vibration-averaged bond lengths 〈R〉 and angle 〈θ〉 were 2.4995 Å and 17.1°, respectively. The ab initio (¹A₁) NaH₂ ⁺ PES yielded a dissociation energy (D ₀) value of 10.3 kJ mol^?1, which is in excellent agreement with the experimental value of 10.3 ± 0.8 kJ mol^?1 (Bushnell et al. in J Phys Chem 98:2044, 1994). An analytical dipole moment surface was constructed using 90 CCSD(T) points. Rovibrational spectra of (¹A₁) NaH₂ ⁺, (¹A′) NaHD⁺ and (¹A₁) NaD₂ ⁺ for v ≤ 10, J ≤ 5 were constructed using rovibrational transition moment matrix elements calculated in a novel manner that employs the analytical dipole moment surface (DMS). The rovibrational structure of the Na⁺–H₂ v _HH = 1 ← v _HH = 0 band was calculated and compared to that of Li⁺–H₂.     

Silaacetylene: A possible target for experimental studies

The equilibrium geometries and transition states for interconversion of the CSiH₂ isomers in the singlet electronic ground state are optimized at the MP2 and CCSD(T) levels of theory using a TZ2P basis set. The heats of formation, vibrational frequencies, infrared intensities, and rotational constants are also predicted. There are three energy minima on the CSiH₂ potential energy surface. Energy calculations at CCSD(T)/TZ2P(fd) + ZPE predict that the global energy minimum is silavinylidene (1), which is 34.1 kcal mol⁻¹ lower in energy than trans-bent silaacetylene (2) and 84.1 kcal mol⁻¹ more stable than the vinylidene isomer (3). The barrier for rearrangement 2→1 is calculated at the same level of theory to be 5.1 kcal mol⁻¹, while for the rearrangement 3→2 a barrier of 2.7 kcal mol⁻¹ is predicted. The natural bond orbital (NBO) population scheme indicates a clear polarization of the C(SINGLE BOND)Si bonds toward the carbon end. A significant ionic contribution to the C(SINGLE BOND)Si bonds of 1 and 2 is suggested by the NBO analysis. The C(SINGLE BOND)Si bond length of trans-bent silaacetylene (2) is longer than previously calculated [1.665 Å at CCSD(T)/TZ2P)]. The calculated carbon-silicon bond length of 2 is in the middle between the C(SINGLE BOND)Si double bond length of 1 (1.721 Å) and the C(SINGLE BOND)Si triple bond of the linear form HCSiH (4), which is 1.604 Å. Structure 4 is a higher-order saddle point on the potential energy surface. © 1996 by John Wiley & Sons, Inc.