Semi-experimental Equilibrium Structure Research Articles

Semiexperimental Equilibrium Structure for the C6 Backbone of cis-1,3,5-Hexatriene; Structural Evidence for Greater π-Electron Delocalization with Increasing Chain Length in Polyenes

Twenty-five microwave lines were observed for cis-1,3,5-hexatriene (0.05 D dipole moment) and a smaller number for its three (13)C isotopomers in natural abundance. Ground-state rotational constants were fitted for all four species to a Watson-type rotational Hamiltonian for an asymmetric top (kappa = -0.9768). Vibration-rotation (alpha) constants were predicted with a B3LYP/cc-pVTZ model and used to adjust the ground-state rotational constants to equilibrium rotational constants. The small inertial defect for cis-hexatriene shows that the molecule is planar, despite significant H-H repulsion. The substitution method was applied to the equilibrium rotational constants to give a semiexperimental equilibrium structure for the C(6) backbone. This structure and one predicted with the B3LYP/cc-pVTZ model show structural evidence for increased pi-electron delocalization in comparison with butadiene, the first member of the polyene series.

Anharmonic force field and equilibrium structure of nitric acid

The quadratic, cubic and semi-diagonal quartic force field of nitric acid has been calculated at the CCSD(T) level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. It is found that the A and B semi-experimental equilibrium rotational constants of the 18O isotopologues (for which the rotation of principal axes is large) cannot be accurately reproduced. This problem is discussed and a remedy is proposed. Finally, the semi-experimental structure is in agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of at least quadruple-ζ quality and a core correlation correction, except for the long N O single bond for which the CCSD(T) value is too short due to inadequate treatment of electron correlation. The empirical structures are also determined and their accuracy is discussed. The best equilibrium structure is: r e(N O syn) = 1.209(1) Å, r e(N O anti) = 1.194(1) Å, r e(N O) = 1.397(1) Å, r e(O H) = 0.968(1) Å, ∠ (ON O syn) = 115.8(1)°, ∠ (ON O anti) = 114.2(1)° and ∠ (NOH) = 102.2(1)°.

High-resolution infrared spectra of the two nonpolar isomers of 1,4-difluorobutadiene

High-resolution (0.0013 cm −1) infrared spectra have been recorded for trans,trans-1,4-difluorobutadiene (ttDFBD) and cis,cis-1,4-difluorobutadiene (ccDFBD). The rotational structure in two C-type bands ( ν 10 and ν 12) and one A-type band ( ν 22) for ttDFBD and in two C-type bands ( ν 11 and ν 12) for ccDFBD has been analyzed. Ground state and upper state rotational constants, except for ν 10 of ttDFBD, have been fitted. Band centers are 934.1 cm −1 ( ν 10), 227.985 cm −1 ( ν 12), and 1087.919 cm −1 ( ν 22) for ttDFBD. Band centers are 762.891 cm −1 ( ν 11) and 327.497 cm −1 ( ν 12) for ccDFBD. The small inertial defects in the ground state confirm that both isomers are planar. Obtaining the ground state rotational constants for the two isomers of DFBD is a first step toward determining their semi-experimental equilibrium structures.

Millimeter-wave spectroscopy of deuterated hydrogen sulfide, SH 2D +

The monodeuterated isotopologue of the sulfonium ion, the asymmetric species SH 2D +, was produced in a magnetically confined negative glow discharge and detected for the first time by means of its rotational spectrum in the region 324–637 GHz. The determined rotational constants, along with those for the symmetric isotopologues and for SHD 2 + obtained in previous measurements, enabled to compute a semi-experimental equilibrium structure, where the contribution of the zero-point vibrations is assumed from available ab initio calculations. In addition, thanks to the large body of quartic centrifugal distortion constants now available, the harmonic force field of the sulfonium ion has been refined by a least-squares procedure.

High-Resolution Infrared Spectroscopy of trans- and cis-H18ON18O: Equilibrium Structures of the Nitrous Acid Isomers

In this paper, we present the first high-resolution spectra and analysis of the nu 4 fundamental bands of fully (18)O-substituted nitrous acid, trans- and cis-H(18)ON(18)O. These bands are not perturbed by neighboring vibrational levels and were used to determine for the first time accurate rotational and centrifugal distortion constants of the ground and nu 4 = 1 states of trans- and cis-H(18)ON(18)O. The ground-state rotational constants were then used, together with the rotational constants of other HONO isotopic species and with rotation-vibration parameters from ab initio calculations, to determine accurate semi-experimental equilibrium structures of trans- and cis-HONO. Our study confirms the results of a recent work by Demaison et al. (J. Phys. Chem. A 2006, 110, 13609-13617) concerning the structure of trans-HONO, whereas the new structure of cis-HONO obtained in this paper is a significant improvement compared with the previous work of Cox et al. (J. Mol. Struct. 1994, 320, 91-106). The recommended parameters for the equilibrium structure of cis-HONO are r(e)(ON) = 1.1816(10) A, r(e)(N-O) = 1.3887(10) A, r(e)(O-H) = 0.9744(7) A, angle(e)(ONO) = 113.18(1) degrees, and angle(e)(HON) = 104.67(4) degrees.

Torsional barrier and equilibrium structure of ethyl cyanide

The quadratic, cubic and semi-diagonal quartic force field of ethyl cyanide has been calculated at the B3LYP level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quadruple-ζ quality and a core correlation correction. The empirical structures are also determined and their accuracy is discussed. The potential barrier V 3 hindering internal rotation of the methyl group has been calculated from 23 rotational transitions of CH 3CH 2C 15N which were found split into doublets, giving V 3 = 3074(27) cal mol −1.

Equilibrium Structure and Torsional Barrier of BH3NH3

Born-Oppenheimer equilibrium structures, r(e)(BO), of the electronic ground state of the borazane (BH3NH3) molecule of C3v point-group symmetry are computed ab initio using the CCSD(T) method with basis sets up to quintuple-zeta quality. Inclusion of the counterpoise correction and extrapolation of the structural parameters to the complete basis set limit yield a best estimate of r(e)(BO) of BH3NH3. The anharmonic force field of BH3NH3, computed at the CCSD(T) level of theory with a basis set of triple-zeta quality, allows the determination of semi-experimental equilibrium rotational constants, which in turn result in a semi-experimental equilibrium structure, r(e)(SE). The r(e)(BO) and r(e)(SE) structures are in excellent agreement, indicating the validity of the methods used for their determination. The empirical mass-dependent structure, r(m)(1), of BH3NH3 is also determined. Although it is inferior in quality to the previous two structures, it is much more accurate than the standard empirical r0 and r(s) structures reported earlier for BH3NH3. The semi-experimental r(e)(SE) as well as the empirical r(m)(1) structures determined are based on experimental ground-state rotational constants available from the literature for nine isotopologues of borazane. The effective barrier to the internal rotation of BH3NH3, a molecule isoelectronic with CH3CH3, has been computed ab initio, employing the focal-point analysis (FPA) approach, to be 699 +/- 11 cm(-1). This compares favorably with an empirical redetermination of the effective barrier based on the above r(e)(SE) structure, V3 = 718(17) cm(-1).

Rotational spectrum of CH 3C 13CCCH: Determination of the equilibrium structure of methyldiacetylene from microwave spectroscopy and ab initio calculations

The rotational spectrum of CH 3C 13CCCH, observed in natural abundance, has been recorded and assigned for the first time. The present investigation, carried out in the millimeter- and submillimeter-wave frequency region, allows us to provide accurate ground state rotational and centrifugal distortion constants. The anharmonic force field of methyldiacetylene has been calculated at the MP2 level of theory employing a basis set of triple-zeta quality. Semi-experimental equilibrium structure has then been derived from the experimental ground-state rotational constants available for various isotopologues and the corresponding vibrational corrections calculated from the ab initio force field. This structure has been found in good agreement with pure theoretical best estimates of equilibrium geometry as well as with previous experimental structures, namely r 0, r s, and r m.

Equilibrium structure of chloroform

The quadratic, cubic and semi-diagonal quartic force field of chloroform has been calculated at the MP2 level of theory employing a basis set of triple-ζ quality and in the frozen core approximation. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and vibration–rotation interaction constants calculated from the ab initio force field determined in rectilinear normal coordinates. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quintuple-ζ quality and a core correlation correction. Experimental mass-dependent r m structures are also determined and are found to be much less accurate.

Ab initio anharmonic force field and equilibrium structure of propene

The quadratic, cubic and semi-diagonal quartic force field of propene has been calculated at the MP2 level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quintuple-ζ quality and a core correlation correction. The experimental mass-dependent r m structures are also determined and their accuracy is discussed. The use of isolated CH stretching frequencies is shown to be a good method to determine CH bond length.

Anharmonic force field of cis- and trans-formic acid from high-level ab initio calculations, and analysis of resonance polyads

The quadratic, cubic, and semidiagonal quartic force fields of cis- and trans-formic acid have been calculated using three different levels of theory. They all give satisfactory results, including the one at the lowest level of theory which is the MP2 method employing a basis set of triple-zeta quality. The results are used to theoretically analyze resonance polyads, including the one involving the 4(1), 5(1), 6(1), 8(1), 7(1)9(1), and 9(2) vibrational states. A semiexperimental equilibrium structure is derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. The ab initio structure calculated at the CCSD(T) level of theory using a basis set of quintuple-zeta quality is in excellent agreement with the semiexperimental structure.

Vibrational Spectroscopy of 1,1-Difluorocyclopropane-d0, -d2, and -d4: The Equilibrium Structure of Difluorocyclopropane

IR and Raman spectra are reported for 1,1-difluorocyclopropane-d0, -d2, and -d4, and complete assignments of vibrational fundamentals are given for these species. These assignments are consistent with predictions of frequencies, intensities, and Raman depolarization ratios computed with the B3LYP/cc-pVTZ quantum chemical (QC) model. Ground state rotational constants for five 13C and deuterium isotopomers, obtained from published microwave spectra, were "corrected" into equilibrium rotational constants. The needed vibration-rotation interaction constants were computed with QC models after scaling the force constants. A semi-experimental equilibrium structure, fitted to the equilibrium moments of inertia, is rC1C = 1.470(1) A, rCC = 1.546(1) A, rCF = 1.343(1) A, rCH = 1.078(1) A, alphaFCF = 109.5(1), alphaFCC = 119.4(1) degrees, alphaHCH = 116.7(1) degrees, alphaC1CH = 117.4(1) degrees, and alphaCCH = 117.1(1) degrees. This structure agrees within the indicated uncertainties with the ab initio structure obtained from an extrapolated set of CCSD(T)/aug-cc-pVnZ calculations except for rCC = 1.548 A. The F2C-CH2 bonds are significantly shortened and strengthened; the H2C-CH2 bond is significantly lengthened and weakened.

Ab initio anharmonic force field and equilibrium structure of the sulfonium ion

The semi-experimental equilibrium structure of the sulfonium ion, SH 3 + , has been obtained from the experimental ground-state rotational constants available for five isotopologues and the corresponding vibrational corrections computed at the CCSD(T)/cc-pwCVQZ level of theory. This geometry has been found in very good agreement with the pure ab initio equilibrium structure calculated at the CCSD(T) level of theory using a basis set of sextuple-zeta quality and including core correlation corrections. The anharmonic force field has been used for deriving spectroscopic properties: in particular, in addition to the vibrational corrections, the rotational parameters of the SH 2D + isotopic species, not yet experimentally observed, have been predicted to a guessed good accuracy.

Equilibrium structure of methylcyanide

The quadratic and cubic force fields of methylcyanide have been calculated at the MP2 and CCSD(T) levels of theory employing a core-valence basis set of triple-zeta quality. Semi-experimental equilibrium structures have then been derived from the experimental ground-state rotational constants available for various isotopologues and the corresponding vibrational corrections calculated from the ab initio force fields. These structures have been found in excellent agreement with the pure ab initio structure calculated at the CCSD(T) level of theory using a basis set of sextuple-zeta quality and including core correlation corrections.

Ab initio anharmonic force field and equilibrium structure of vinyl fluoride and vinyl iodide

The quadratic, cubic and semi-diagonal quartic force field of vinyl fluoride and vinyl iodide have been calculated at the MP2 level of theory employing a basis set of triple- ζ quality including a relativistic pseudopotential on iodine. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quadruple- ζ quality. Finally, the structure of different vinyl derivatives is compared.

Ab initio anharmonic force field and equilibrium structure of vinyl bromide

The quadratic, cubic, and semi-diagonal quartic force field of vinyl bromide has been calculated at the MP2 level of theory employing a basis set of triple-ζ quality including a relativistic pseudopotential on bromine. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quadruple-ζ quality and an offset correction. The experimental mass-dependent r m structures are also determined and their accuracy is discussed.

Anharmonic force field, structure, and thermochemistry of CF2 and CCl2Electronic supplementary information (ESI) available: Force constants and thermochemical properties of CF2 and CCl2. See http://www.rsc.org/suppdata/cp/b2/b202865d/

The anharmonic force field up to quartic terms has been calculated for both CF2 and CCl2 at the CCSD(T) level of theory with large basis sets. The calculated spectroscopic parameters are in excellent agreement with the available experimental data. The equilibrium structure of CCl2 has been calculated at the CCSD(T) level of theory with the cc-pV5Z basis set (including core correlation corrections). It is in excellent agreement with the semi-experimental equilibrium structure derived from the experimental rotational constants and the ab initio rovibrational interaction parameters. The heats of formation of CF2 and CCl2 have been calculated using the W2 method and are −193.43 and 227.96 kJ mol−1, respectively.