Microwave spectrum of dimethylallene excited states and strong Coriolis coupling

The rotational spectra of six excited vibrational states of dimethylallene were measured and assigned to the corresponding vibrational levels, and for three more excited state spectra at least the rotational constants could be determined. Between the two lowest excited levels of symmetry species b₂ and b₁ of group C_2v a strong a-type Coriolis coupling was found to exist. The evaluation of the resulting perturbation by a diagonalization of the energy matrix yielded ζ^(a) = 0.36 and a precise value for the vibrational energy difference 48.761 GHz (1.6 cm^?1). The state b₂ is believed to be the first excited torsional substate (01, 10)₁ of methyl internal rotation, and the rotational transitions of this state as well as those of the strongly coupled state b₁ presented very irregular multiplet splittings. On the other hand, the splittings of the next-higher excited state of species a₂ which could be identified as the partner torsional substate (01, 10)₂, followed the regular pattern, yielding an internal rotation barrier V₃ (2079 cal/mole) not unlike that derived earlier from ground state splittings.     

Microwave spectrum of dimethylketene: Centrifugal distortion and Coriolis interaction

For the prolate asymmetric (κ = ?0.58) top molecule dimethylketene the centrifugal constants have been determined in terms of the Δ constants by means of ΔK_? ≠ 0 rotational transitions. Two earlier assigned torsional excited states (01, 10)₁ and (01, 10)₂ of symmetry B₂ and A₂ of C_2v could be confirmed. A third fundamental vibration v_r(b₁), probably the inplane skeletal rock, has been found and analyzed in the v_r = 1 and v_r = 2 states. The variation of the centrifugal constants and of the inertial defects with the state of excitation could be explained by Coriolis coupling between the v_r(b₁) = 1 vibrational and v_t(b₂) = 1 torsional excited states. The Coriolis constant $\text{ζ}{}_{\mathrm{rt}}{}^{\mathrm{a}}$ and the torsional frequency ω_t(b₂) have been estimated to be $\text{ζ}{}_{\mathrm{rt}}{}^{\mathrm{a}}\text{≈ 0.24}$ and ω_r(b₁) - ω_t(b₂) ≈ 5.8 cm^?1, respectively.     

Structural and conformational properties of 2-propenylgermane (allylgermane) studied by microwave and infrared spectroscopy and quantum chemical calculations

The structural and conformational properties of allylgermane have been investigated using Stark and Fourier transform microwave spectroscopies, infrared spectroscopy, and high-level quantum chemical calculations. The parent species H2C=CHCH2GeH3 was investigated by microwave spectroscopy and infrared spectroscopy, while three deuterated species, namely, H2C=CDCH2GeH3, H2C=CHCHDGeH3, and H2C=CHCH2GeD3, were studied only by infrared spectroscopy. The microwave spectra of the ground vibrational state as well as of the first excited state of the torsion vibration around the sp2-sp3 carbon-carbon bond were assigned for the 70Ge, 72Ge, and 74Ge isotopomers of one conformer. This rotamer has an anticlinal arrangement for the C=C-C-Ge chain of atoms. The infrared spectrum of the gas in the 500-4000 cm(-1) range has been assigned. No evidence of additional rotameric forms other than anticlinal was seen in the microwave and infrared spectra. Several different high-level ab initio and density functional theory calculations have been performed. These calculations indicate that a less stable form, having a synperiplanar conformation of the C=C-C-Ge link of atoms, may coexist with the anticlinal form. The energy differences between the synperiplanar and anticlinal forms were calculated to be 5.6-9.2 kJ/mol depending on the computational procedure. The best approximation of the equilibrium structure of the anticlinal rotamer was found in the MP2/aug-cc-pVTZ calculations. The barrier to internal rotation of the germyl group was found to be 6.561(17) kJ/mol, from measurements of the splitting of microwave transitions caused by tunneling of the germyl group through its threefold barrier.     

Doppler-free rotational spectrum of methyl iodide. Nuclear quadrupole,spin-rotation and nuclear shielding tensors of iodine

The ground-state microwave spectrum of methyl iodide has been measured between 59 and 240 GHz with a molecular beam spectrometer and by use of the Lamb dip method. The following molecular parameters have been accurately determined: B = 7501275.70(2) kHz; D_J = 6.3070(2) kHz; D_JK = 98.762(3) kHz; H_J = ?0.0051(6) Hz; H_JK = 0.04(2) Hz; H_KJ 4.51(3) Hz; eq. Q = ?1934.136 (5) MHz; C_⊥= ?17.40(6) kHz; C_| = ?19.4(5) kHz; x_J = ?1.41(4) kHz; x_K = ?38(1) kHz and x_d = 26.2(6) kHz. The last three constants characterize the interaction of centrifugal distortion with quadrupole coupling. The data are used to determine the elements of the iodine nuclear magnetic shielding tensor. Comparisons of the shielding are made for molecules which contain one iodine atom.     

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).     

Equilibrium vs ground-state planarity of the CONH linkage

Planarity of the XC(=)NHY linkage has been investigated in unprecedented detail in a number of relatively simple compounds, including formamide (X = Y = H), acetamide (X = CH3, Y = H), urea (X = NH2, Y = H), carbamic acid (X = OH, Y = H), and methyl carbamate (X = OCH3, Y = H). Reliable estimates of the equilibrium structures of formamide, cyanamide, acetamide, urea, carbamic acid, methylamine, dimethyl ether, and methyl carbamate are derived, mostly for the first time. It is shown that formamide, considered prototypical for the amide linkage, is not typical as it has a planar equilibrium amide linkage corresponding to a single-minimum inversion potential around N. In contrast, several molecules containing the CONH linkage seem to have a pyramidalized nitrogen at equilibrium and a double-minimum inversion potential with a very small inversion barrier allowing for an effectively planar ground-state structure. Observables of rotational spectroscopy, including ground-state inertial defects, quadrupole coupling and centrifugal distortion constants, and dipole moment components, as well as equilibrium C=O and C-N bond lengths are reviewed in their ability to indicate the planarity of the effective and possibly the equilibrium structures.