High-accuracy structure of cyclobutane by femtosecond rotational Raman four-wave mixing

The femtosecond degenerate four-wave mixing (fs-DFWM) technique is applied for the measurement of accurate rotational constants of cyclobutane (C4H8). The vibrational levels of C4H8 exhibit tunneling splitting due to the ring-puckering interconversion between the symmetry-equivalent D2d minima via a planar D4h barrier. For the v = 0 ground state, the fs-DFWM method yields a rotational constant B + 0 = 10663.452(18) MHz. The ring-puckering tunneling leads to slightly different rotational constants for the 0+ and 0- levels, B + 0 - B -0 = 33 +/- 2 kHz. This difference increases by a factor of approximately 90 in the v = 1+/1- ring-puckering states to B +1 - B -1 = -3059 +/- 4 kHz. Combining the experimental rotational constants with the structure parameters and rotational constants calculated by high-level ab initio calculations allows us to determine accurate equilibrium and vibrationally averaged structure parameters for cyclobutane, for example, re(C-C) = 1.5474 A, re(C-Haxial) = 1.0830 A, re(C-Hequatorial) = 1.0810 A, and ring puckering angle theta e = 29.8 degrees .     

Femtosecond degenerate four-wave mixing of cyclopropane

Femtosecond degenerate four-wave mixing (fs-DFWM) is applied for the measurement of rotational constants of cyclopropane (C3H6). The rotational coherence method yields a very accurate B0 = 20,093.322(12) MHz and centrifugal distortion constants D(J) and D(JK). To exploit the full resolution of the fs-DFWM method, the accuracy of the optical delay measurement was increased by nearly two orders of magnitude, including elimination of effects from the refractive index of air. The fs-DFWM molecular constants are comparable in accuracy to those from high-resolution infrared spectroscopy and are only surpassed by those of dipole distortion microwave spectroscopy. In parallel, the equilibrium structure, vibrationally averaged structure parameters and rotational constants were calculated using high-level ab initio methods and large basis sets. Combining these with the results of previous calculations and the measured rotational constants yields r(e)(C-C) = 1.5034(3) A, r(e)(C-H) = 1.0775(5) A, and alpha(e)(H-C-H) = 115.09(10) degrees.     

Femtosecond rotational Raman coherence spectroscopy of cyclohexane in a pulsed supersonic jet

We combine the technique of femtosecond degenerate four-wave mixing (fs-DFWM) with a high repetition-rate pulsed supersonic jet source to obtain the rotational coherence spectrum (RCS) of cold cyclohexane (C(6)H(12)) with high signal/noise ratio. In the jet expansion, the near-parallel flow pattern combined with rapid translational cooling effectively eliminate dephasing collisions, giving near-constant RCS signal intensities over time delays up to 5 ns. The vibrational cooling in the jet eliminates the thermally populated vibrations that complicate the RCS coherences of cyclohexane at room temperature [Brügger, G.; et al. J. Phys. Chem. A 2011, 115, 9567]. The rotational cooling reduces the high-J rotational-state population, yielding the most accurate ground-state rotational constant to date, B(0) = 4305.859(9) MHz. Based on this B(0), a reanalysis of previous room-temperature gas-cell RCS measurements of cyclohexane gives improved vibration-rotation interaction constants for the ν(32), ν(6), ν(16), and ν(24) vibrational states. Combining the experimental B(0)(C(6)H(12)) with CCSD(T) calculations yields a very accurate semiexperimental equilibrium structure of the chair isomer of cyclohexane.     

Accurate determination of the structure of cyclohexane by femtosecond rotational coherence spectroscopy and ab initio calculations

We combine femtosecond time-resolved rotational coherence spectroscopy with high-level ab initio theory to obtain accurate structural information for the nonpolar molecules cyclohexane (C(6)H(12)) and cyclohexane-d(12) (C(6)D(12)). We measured the rotational B(0) and centrifugal distortion constants D(J), D(JK) of the v = 0 states of C(6)H(12) and C(6)D(12) to high accuracy, for example, B(0)(C(6)H(12)) = 4306.08(5) MHz, as well as B(v) for the vibrationally excited states ν(32), ν(6), ν(16) and ν(24) of C(6)H(12) and additionally ν(15) for C(6)D(12). To successfully reproduce the experimental RCS transient, the overtone and combination levels 2ν(32), 3ν(32), ν(32) + ν(6), and ν(32) + ν(16) had to be included in the RCS model calculations. The experimental rotational constants are compared to those obtained at the second-order M?ller-Plesset (MP2) level. Combining the experimental and calculated rotational constants with the calculated equilibrium bond lengths and angles allows determination of accurate semiexperimental equilibrium structure parameters, for example, r(e)(C-C) = 1.526 ± 0.001 ?, r(e)(C-H(axial)) = 1.098 ± 0.001 ?, and r(e)(C-H(equatorial)) = 1.093 ± 0.001 ?. The equilibrium C-C bond length of C(6)H(12) is only 0.004 ? longer than that of ethane. The effect of ring strain due to the unfavorable gauche interactions is mainly manifested as small deviations from the C-C-C, C-C-H(axial), and C-C-H(equatorial) angles from the tetrahedral value.     

Accurate determination of the structure of cyclooctatetraene by femtosecond rotational coherence spectroscopy and ab initio calculations

We combine femtosecond time-resolved rotational coherence spectroscopy with high-level ab initio theory to obtain accurate structural information for the nonpolar antiaromatic molecule 1,3,5,7-cyclooctatetraene (C8H8, COT) and its perdeuterated isotopomer COT-d8 (C8D8). We measure the rotational B0 and centrifugal distortion constants D(J), D(JK) of the v = 0 states of COT and COT-d8 to high accuracy, e.g. B0 (COT) = 2710.329(56) MHz, as well as B(v) for the v = 1 states nu6, nu11, nu17, nu22, and nu41/nu42 of COT. The experimental rotational constants are compared to those obtained from calculations at the coupled-cluster with single, double, and perturbative triples [CCSD(T)] level. The latter also take into account vibrational averaging effects of the ground and vibrationally excited states. Combining the experimental and calculated rotational constants with the calculated equilibrium bond lengths and angles allows us to determine accurate equilibrium structure parameters, e.g., r(e) (C-C) = 147.0 +/- 0.05 pm, r(e) (C=C) = 133.7 +/- 0.1 pm, and r(e) (C-H) = 107.9 +/- 0.1 pm. The equilibrium C-C and C=C bond lengths of COT are compared to those of 1,3-butadiene. The expected effect of decreased pi-electron delocalization due to the twisting of adjacent C=C double bonds in COT relative to butadiene is observed for the C-C bonds but not for the C=C bonds.     

The Al+ -H(2) cation complex: rotationally resolved infrared spectrum, potential energy surface, and rovibrational calculations

The infrared spectrum of the Al(+)-H(2) complex is recorded in the H-H stretch region (4075-4110 cm(-1)) by monitoring Al(+) photofragments. The H-H stretch band is centered at 4095.2 cm(-1), a shift of -66.0 cm(-1) from the Q(1)(0) transition of the free H(2) molecule. Altogether, 47 rovibrational transitions belonging to the parallel K(a)=0-0 and 1-1 subbands were identified and fitted using a Watson A-reduced Hamiltonian, yielding effective spectroscopic constants. The results suggest that Al(+)-H(2) has a T-shaped equilibrium configuration with the Al(+) ion attached to a slightly perturbed H(2) molecule, but that large-amplitude intermolecular vibrational motions significantly influence the rotational constants derived from an asymmetric rotor analysis. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 3.03 A, decreasing by 0.03 A when the H(2) subunit is vibrationally excited. A three-dimensional potential energy surface for Al(+)-H(2) is calculated ab initio using the coupled cluster CCSD(T) method and employed for variational calculations of the rovibrational energy levels and wave functions. Effective dissociation energies for Al(+)-H(2)(para) and Al(+)-H(2)(ortho) are predicted, respectively, to be 469.4 and 506.4 cm(-1), in good agreement with previous measurements. The calculations reproduce the experimental H-H stretch frequency to within 3.75 cm(-1), and the calculated B and C rotational constants to within approximately 2%. Agreement between experiment and theory supports both the accuracy of the ab initio potential energy surface and the interpretation of the measured spectrum.     

Semispectroscopic and quantitative structure-property relationship estimates of the equilibrium and vibrationally averaged structure and dipole moment of 1-buten-3-yne

Systematic quantum chemical calculations have been performed to obtain precise estimates of the equilibrium and vibrationally averaged molecular structure and electric dipole moment of vinylacetylene (VA, 1-buten-3-yne). Anharmonic (cubic and semi-diagonal quartic) MP2/cc-pVTZ force fields in normal coordinates were computed to account for anharmonic vibrational effects, including zero-point contributions to the rotational constants and the electric dipole moment. A simultaneous weighted least-squares structural refinement was performed, resulting in the best semispectroscopic estimate of the re structure of VA. The refinement was based on experimentally measured ground-state rotational constants of two isotopologs of VA corrected to equilibrium values using MP2/cc-pVTZ vibration-rotation interaction constants and all-electron CCSD(T)/aug-cc-pVTZ structural constraints. The semispectroscopic re structure of VA agrees excellently with the high-level CCSD(T)/aug-cc-pVTZ ab initio structure. The most dependable, CCSD(T)/cc-pVQZ//CCSD(T)/aug-cc-pVTZ equilibrium electric dipole moment of VA, in D, is mua= 0.4088, mub= 0.0004, and muc= 0. The vibrationally corrected a-component of 0.4214 D is in excellent agreement with one of the available experimental values. The present analysis shows that mub is negligible even after vibrational correction. A simple quantitative structure-property relationship (QSPR) model resulted in a highly similar estimate, 0.45 D, for the electric dipole moment of VA.     

Protonated carbonyl sulfide: prospects for the spectroscopic observation of the elusive HSCO+ isomer

Spurred by the apparent conflict between ab initio predictions and infrared spectroscopic evidence regarding the relative stability of isomers of protonated carbonyl sulfide, key stationary points on the isomerization surface of HOCS(+) have been examined via systematic extrapolations of ab initio energies. Electron correlation has been accounted for using second-order M?ller-Plesset perturbation theory and coupled cluster theory through triple excitations [CCSD, CCSD(T), and CCSDT] in conjunction with the correlation consistent hierarchy of basis sets, cc-pVXZ (X=D,T,Q,5,6). HSCO(+) is predicted to lie lower in energy than HOCS(+) by 4.86 kcal mol(-1), computed using the focal point extrapolation scheme of Allen and co-workers [J. Chem. Phys. 99, 4638 (1993)] with corrections for anharmonic zero-point vibrational energy, core correlation, non-Born-Oppenheimer, and scalar relativistic effects. A transition state has been located, constituting the barrier to isomerization of HSCO(+) to HOCS(+), lying 68.9 kcal mol(-1) higher in energy than HSCO(+). This is well above predicted exothermicity [DeltaH(r) (o)(0 K)=48.1 kcal mol(-1), cc-pVQZ CCSD(T)] for the reaction considered in the experiments (HSCO(+)+H(2)-->OCS+H(3) (+)). Though proton tunneling will lead to a lower effective barrier, this prediction is consistent with the lack of HSCO(+) in electrical discharges in H(2)OCS, since the relative populations of HOCS(+) and HSCO(+) will depend on the experimental details of the protonation route rather than the relative thermodynamic stability of the isomers. Anharmonic vibrational frequencies and vibrationally corrected rotational constants from cc-pVTZ CCSD(T) cubic and quartic force constants are provided, to aid in the spectroscopic observation of the energetically favorable but apparently elusive HSCO(+) isomer.     

Microwave spectra and gas phase structural parameters for N-hydroxypyridine-2(1H)-thione

The microwave spectrum for N-hydroxypyridine-2(1H)-thione (pyrithione) was measured in the frequency range 6-18 GHz, providing accurate rotational constants and nitrogen quadrupole coupling strengths for three isotopologues, C(5)H(4)(32)S(14)NOH, C(5)H(4)(32)S(14)NOD, and C(5)H(4)(34)S(14)NOH. Pyrithione was found to be in a higher concentration in the gas phase than the other tautomer, 2-mercaptopyridine-N-oxide (MPO). Microwave spectroscopy is best suited to determine which structure predominates in the gas phase. The measured rotational constants were used to accurately determine the coordinates of the substituted atoms and provided sufficient data to determine some of the important structural parameters for pyrithione, the only tautomer observed in the present work. The spectra were obtained using a pulsed-beam Fourier transform microwave spectrometer, with sufficient resolution to allow accurate measurements of the (14)N nuclear quadrupole hyperfine interactions. Ab initio calculations provided structural parameters and quadrupole coupling strengths that are in very good agreement with measured values. The experimental rotational constants for the parent compound are A = 3212.10(1), B = 1609.328(7), and C = 1072.208(6) MHz, yielding the inertial defect Δ(0) = -0.023 amu·?(2) for the C(5)H(4)(32)S(14)NOH isotopologue. The observed near zero inertial defect clearly indicates a planar structure. The least-squares fit structural analysis yielded the experimental bond lengths R(O-H) = 0.93(2) ?, R(C-S) = 1.66(2) ?, and angle (N-O-H) = 105(4)° for the ground state structure.     

An investigation of the molecular geometry and electronic structure of nitryl chloride by a combination of rotational spectroscopy and ab initio calculations

The ground-state rotational spectra of the six isotopomers (16)O(2) (14)N(35)Cl, (16)O(2) (14)N(37)Cl, (18)O(16)O(14)N(35)Cl, (18)O(2) (14)N(35)Cl, (16)O(2) (15)N(35)Cl, and (16)O(2) (15)N(37)Cl of nitryl chloride were observed with a pulsed-jet, Fourier-transform microwave spectrometer to give rotational constants, Cl and (14)N nuclear quadrupole coupling, and spin-rotation coupling constants. These spectroscopic constants were interpreted to give r(0), r(s), and r(m) ((2)) versions of the molecular geometry and information about the electronic redistribution at N when nitryl chloride is formed from NO(2) and a Cl atom. The r(m) ((2)) geometry has r(N-Cl)=1.8405(6) A, r(N-O)=1.1929(2) A, and the angle ONO=131.42(4) degrees , while the corresponding quantities for the r(s) geometry are 1.8489 A, 1.1940 A, and 131.73 degrees , respectively. Electronic structure calculations at CCSD(T)cc-pVXZ (X=T, Q, or 5) levels of theory were carried out to give a r(e) geometry, vibration-rotation corrections to equilibrium rotational constants, and values of the (35)Cl and (14)N nuclear hyperfine (quadrupole and spin-rotation) coupling constants in good agreement with experiment. The equilibrium geometry at the CCSD(T)/cc-pV5Z level of theory has r(N-Cl)=1.8441 A, r(N-O)=1.1925 A and the angle ONO=131.80 degrees . The observed rotational constants were corrected for the vibration-rotation effects calculated ab initio to yield semiempirical equilibrium constants which were then fitted to give the following semiempirical equilibrium geometry: r(N-Cl)=1.8467(2) A, r(N-O)=1.1916(1) A, and the angle ONO=131.78(3) degrees .     

Fourier transform microwave spectroscopy of HSiBr: exploring the Si-Br bond through quadrupole hyperfine coupling

The 1(01)-0(00) (9-10 GHz) and 2(02)-1(01) (18-19 GHz) rotational transitions of HSi 79Br and HSi 81Br have been measured in a pulsed discharge jet expansion to an experimental uncertainty of approximately 1 kHz using Fourier transform microwave spectroscopy. The data have yielded an effective rotational constant, the centrifugal distortion constant Dj, the bromine nuclear quadrupole coupling constants, and the bromine nuclear spin-molecular rotation interaction parameter for both isotopomers. The derived parameters have been compared to their values calculated ab initio, and the nuclear quadrupole coupling tensor has been used to investigate the Si-Br bond, giving a sigma bond ionic character of 0.60, a pi bond character of 0.22, and a total Si-Br ionic character of 0.38. These bond characteristics have been compared to trends in other halosilylenes, silanes, and the analogous carbenes.     

A priori predictions of the rotational constants for protonated formaldehyde and protonated methanol

Protonated formaldehyde and protonated methanol are candidate interstellar molecules and models for classes of protonated oxygen compounds. Ab initio molecular orbital theory has been used to compute rotational constants to guide spectroscopic searches both in the laboratory and in space. The ab initio results are empirically corrected to account for systematic deficiencies in the theory and zero-point vibrational effects; they are expected to be accurate to approximately +/-2%. For H2COH+ the resultant constants are (in GHz) A = 194.3, B = 34.28, and C = 29.14; for H3COH2+ A = 103.7, B = 21.18, and C = 20.30.     

Molecular structure and conformational composition of 1,3-dihydroxyacetone studied by combined analysis of gas-phase electron diffraction data, rotational constants, and results of theoretical calculations. Ideal gas thermodynamic properties of 1,3-dihydroxyacetone

The molecular structure of 1,3-dihydroxyacetone (DHA) has been studied by gas-phase electron diffraction (GED), combined analysis of GED and microwave (MW) data, ab initio, and density functional theory calculations. The equilibrium re structure of DHA was determined by a joint analysis of the GED data and rotational constants taken from the literature. The anharmonic vibrational corrections to the internuclear distances (re-ra) and to the rotational constants (B(i)e-B(i)0) needed for the estimation of the re structure were calculated from the B3LYP/cc-pVTZ cubic force field. It was found that the experimental data are well reproduced by assuming that DHA consists of a mixture of three conformers. The most stable conformer of C2v symmetry has two hydrogen bonds, whereas the next two lowest energy conformers (Cs and C1 symmetry) have one hydrogen bond and their abundance is about 30% in total. A combined analysis of GED and MW data led to the following equilibrium structural parameters (re) of the most abundant conformer of DHA (the uncertainties in parentheses are 3 times the standard deviations): r(C=O)=1.215(2) A, r(C-C)=1.516(2) A, r(C-O)=1.393(2) A, r(C-H)=1.096(4) A, r(O-H)=0.967(4) A, angleC-C=O=119.9(2) degrees, angleC-C-O=111.0(2) degrees, angleC-C-H=108.2(7) degrees, angleC-O-H=106.5(7) degrees. These structural parameters reproduce the experimental B(i)0 values within 0.05 MHz. The experimental structural parameters are in good agreement with those obtained from theoretical calculations. Ideal gas thermodynamic functions (S degrees (T), C degrees p(T), and H degrees (T)-H degrees (0)) of DHA were calculated on the basis of experimental and theoretical molecular parameters obtained in this work. The enthalpy of formation of DHA, -523+/-4 kJ/mol, was calculated by the atomization procedure using the G3X method.     

Rotational spectra and computational analysis of two conformers of leucinamide

Rotational spectra were recorded for two isotopic species of two conformers of the amide derivative of leucine in the range of 10.5-21 GHz and fit to a rigid rotor Hamiltonian. Ab initio calculations at the MP2/6-311++G(d,p) level identified the low energy conformations with different side chain configurations; the rotational spectra were assigned to the two lowest energy ab initio structures. We recorded 16 a- and b-type rotational transitions for conformer 1; the rotational constants of the normal species are A = 2275.6(2), B = 1033.37(2) and C = 911.71(5) MHz. We recorded 23 a- and b-type rotational transitions for conformer 2; the rotational constants of the normal species are A = 2752.775(8), B = 843.502(1) and C = 796.721(1) MHz. The rotational spectra of the (15)N(amide) isotopomer of each conformer were recorded and the atomic coordinates of the amide nitrogen were determined by Kraitchman's method of isotopic substitution. The experimentally observed structures are significantly different from the crystal structures of leucinamide and the gas-phase structures of leucine, and a natural bond orbital analysis revealed the donor-acceptor interactions governing side chain configuration.     

Rotational spectra and structure of the Ar2-H2S complex: pulsed nozzle Fourier transform microwave spectroscopic and ab initio studies

This paper reports the rotational spectrum and structure of the Ar2-H2S complex and its HDS and D2S isotopomers. The ground state structure has heavy-atom C2v symmetry with the two Ar atoms indistinguishable and H2S freely rotating as evinced by the fact that asymmetric top energy levels with Kp=odd levels are missing. The rotational constants for the parent isotopomer are: A=1733.115(1) MHz, B=1617.6160(5) MHz and C=830.2951(2) MHz. Unlike the Ar-H2S complex, the Ar2-H2S does not show an anomalous isotopic shift in rotational constants on deuterium substitution. However, the intermolecular potential is still quite floppy, leading to very different centrifugal distortion constants for the three isotopomers. The Ar-Ar and Ar-c.m.(H2S) distances are determined to be 3.820 A and 4.105 A, respectively. The A rotational constants for Ar2-H2S/HDS/D2S isotopomers are very close to each other and to the B constant of free Ar2, indicating that H2S does not contribute to the moment of inertia about the a-axis. Ab initio calculations at MP2 level with aug-cc-pVQZ basis set lead to an equilibrium C2v minimum structure with the Ar-Ar line perpendicular to the H-H line and the S away from Ar2. The centrifugal distortion constants, calculated using the ab initio force field, are in reasonable agreement with the experimental values. However, they do not show the variation observed for different isotopmers. The binding energy of Ar2-H2S has been determined to be 507 cm-1(6.0 kJ mol-1) by CBS extrapolation after correcting for basis set superposition error. Potential energy scans point out that the barrier for internal rotation of H2S about its b axis is only 10 cm-1 and it is below the zero point energy (13.5 cm-1) in this torsional degree of freedom. Internal rotation of H2S about its a- and c-axes also have small barriers of about 50 cm-1 only, suggesting that H2S is extremely floppy within the complex.     

Synthesis, microwave spectrum, and dipole moment of allenylisocyanide (H2C═C═CHNC), a compound of potential astrochemical interest

An improved synthesis of a compound of potential astrochemical interest, allenylisocyanide (H(2)C═C═CHNC), is reported together with its microwave spectrum, which has been investigated in the 8-120 GHz spectral range to facilitate a potential identification in interstellar space. The spectra of the ground vibrational state and of five vibrationally excited states belonging to three different vibrational modes have been assigned for the parent species. A total of 658 transitions with a maximum value of J = 71 were assigned for the ground state and accurate values obtained for the rotational and quartic centrifugal distortion constants. The spectra of five heavy-atom ((13)C and (15)N) isotopologues were also assigned. The dipole moment was determined to be μ(a) = 11.93(16) × 10(-30) C m, μ(b) = 4.393(44) × 10(-30) C m, and μ(tot) = 12.71(16) × 10(-30) C m. The spectroscopic work has been augmented by theoretical calculations at the CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory. The theoretical calculations are generally in good agreement with the experimental results.     

The microwave spectrum of the 1,1-difluoroprop-2-ynyl radical, F2*C-C[triple bond]CH

The rotational spectrum of the 1,1-difluoroprop-2-ynyl radical, F2*C-C[triple bond]CH, a partially fluorinated variant of the propargyl radical, has been recorded in the ground electronic, 2B1, state using pulsed discharge, pulsed-jet, Fabry-Perot Fourier transform microwave spectroscopy. Five successive a-type rotational transitions, from N = 1-0 to N = 5-4, and Ka = 0, 1, and 2, were measured between 6.5 and 32.5 GHz with an uncertainty of 5 kHz. The molecular constants, including fine and hyperfine constants, were precisely determined. These constants are compared with our predictions based on a density functional theory level ab initio calculations and with the fine and hyperfine constants of the propargyl radical. The measured electron spin densities suggest that both the difluoropropargyl and the difluoroallenyl resonance forms [F2*C-C[triple bond]CH<-->F2C=C=C*H] make major contributions to the electronic structure of the radical.     

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.     

Molecular structures of the two most stable conformers of free glycine

The equilibrium molecular structures of the two lowest-energy conformers of glycine, Gly-Ip and Gly-IIn, have been characterized by high-level ab initio electronic structure computations, including all-electron cc-pVTZ CCSD(T) geometry optimizations and 6-31G* MP2 quartic force fields, the latter to account for anharmonic zero-point vibrational effects to isotopologic rotational constants. Based on experimentally measured vibrationally averaged effective rotational constant sets of several isotopologues and our ab initio data for structural constraints and zero-point vibrational shifts, least-squares structural refinements were performed to determine improved Born-Oppenheimer equilibrium (r(e)) structures of Gly-Ip and Gly-IIn. Without the ab initio constraints even the extensive set of empirical rotational constants available for 5 and 10 isotopologues of Gly-Ip and Gly-IIn, respectively, cannot satisfactorily fix their molecular structure. Excellent agreement between theory and experiment is found for the rotational constants of both conformers, the rms residual of the final fits being 7.8 and 51.6 kHz for Gly-Ip and Gly-IIn, respectively. High-level ab initio computations with focal point extrapolations determine the barrier to planarity separating Gly-IIp and Gly-IIn to be 20.5 +/- 5.0 cm(-1). The equilibrium torsion angle tau(NCCO) of Gly-IIn, characterizing the deviation of its heavy-atom framework from planarity, is (11 +/- 2) degrees. Nevertheless, in the ground vibrational state the effective structure of Gly-IIn has a plane of symmetry.     

Experimental and theoretical studies of the CN-Ar van der Waals complex

The CN-Ar van der Waals complex has been observed using the B (2)Sigma(+)-X (2)Sigma(+) and A (2)Pi-X (2)Sigma(+) electronic transitions. The spectra yield a dissociation energy of D(0")=102+/-2 cm(-1) and a zero-point rotational constant of B(0")=0.067+/-0.005 cm(-1) for CN(X)-Ar. The dissociation energy for CN(A)-Ar was found to be D(0')=125+/-2 cm(-1). Transitions to vibrationally excited levels of CN(B)-Ar dominated the B-X spectrum, indicative of substantial differences in the intermolecular potential energy surfaces (PESs) for the X and B states. Ab initio PESs were calculated for the X and B states. These were used to predict rovibrational energy levels and van der Waals bond energies (D(0")=115 and D(0')=183 cm(-1)). The results for the X state were in reasonably good agreement with the experimental data. Spectral simulations based on the ab initio potentials yielded qualitative insights concerning the B-X spectrum, but the level of agreement was not sufficient to permit vibronic assignment. Electronic predissociation was observed for both CN(A)-Ar and CN(B)-Ar. The process leading to the production of CN(A,nu=8,9) fragments from the predissociation of CN(B,nu=0)-Ar was characterized using time-resolved fluorescence and optical-optical double resonance measurements.