Ab initio and equilibrium bond angles. Structures of HNO and H 2 O 2

Interplay of experiment and theory: high resolution infrared spectrum and accurate equilibrium structure of BF2OH

Substitution of hydrogen by deuterium induces a large rotation of the principal axis system that amplifies the errors.

Gas phase structures of peroxides: experiments and computational problems

Gas-phase structures of several organic and inorganic peroxides X-O-O-X and X-O-O-X', which have been determined experimentally by gas electron diffraction and/or microwave spectroscopy, are discussed. The OO bond length in these peroxides varies from 1.481(8) Å in Me3 SiOOSiMe3 to 1.214(2) Å in FOOF and the dihedral angle ϕ(XO-OX) between 0° in HC(O)O-OH and near 180° in Bu(t) O-OBu(t) . Some of the peroxides cause problems for quantum chemistry, since several computational methods fail to reproduce the experimental structures. Extreme examples are MeO-OMe and FO-OF. In the case of MeO-OMe only about half of the more than 100 computational methods reported in the literature reproduce the experimentally determined double-minimum shape of the torsional potential around the OO bond correctly. For FO-OF only a small number of close to 200 computational methods reproduce the OO and OF bond lengths better than ±0.02 Å.

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.

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.

Structural and Conformational Properties of 1,2-Propadienylphosphine (Allenylphosphine) Studied by Microwave Spectroscopy and Quantum Chemical Calculations

1,2-Propadienylphosphine (allenylphosphine), H(2)C=C=CHPH(2), has been investigated by Stark and Fourier transform microwave spectroscopy. Two rotameric forms denoted syn and gauche have been assigned. The syn form has a symmetry plane (C(s)() symmetry) where the lone electron pair of phosphorus points toward the double bonds. The phosphino group is rotated roughly 120 degrees from this position in the gauche rotamer. The dipole moment of syn was determined to be mu(a) = 1.613(23), mu(b) = 2.347(24), mu(c) = 0 (for symmetry reasons), and mu(tot) = 2.848(28) x 10(-30) C m [0.854(8) D]. The energy difference between the two forms was found to be 2.1(4) kJ/mol from relative intensity measurements with syn as the more stable conformer. Extensive quantum chemical calculations have been carried out and accurate equilibrium structures have been determined for these two rotamers, as well as for the corresponding two conformers of vinylphosphine (H(2)C=CHPH(2)).

The Submillimeter-Wave Spectrum and Quantum Chemical Calculations of Glyoxylic Acid

Glyoxylic acid is a possible candidate for interstellar detection. Many transitions of the submillimeter wave spectrum of the ground vibrational state of its most stable conformer have been measured for the first time. These transitions have been used together with microwave transitions measured previously to obtain accurate spectroscopic constants that should facilitate a search for this compound in interstellar space. High-level quantum chemical calculations of the structure, quartic centrifugal distortion constants, inertial defect, and energy difference between the two low-energy conformers of glyoxylic acid have also been made. Accurate predictions of the equilibrium structures of the most stable forms of glyoxylic, as well as of formic acid, are reported. Copyright 2001 Academic Press.

Experimental and ab Initio Equilibrium Structure and Harmonic Force Field of 1,2,5-Oxadiazole

The equilibrium structure of 1,2,5-oxadiazole has been calculated ab initio at the CCSD(T) level using a polarized valence quadruple zeta basis set. The harmonic force field has also been calculated at the MP2/cc-pVTZ, B3LYP/6-311++G(3df, 2pd), and B3LYP/cc-pVQZ levels. These force fields have been subsequently scaled and further refined by fitting them to the experimental values of the vibrational fundamentals of three isotopomers and the centrifugal distortion constants of the parent molecule. The specific refinement of those scaled force constants particularly sensitive to the experimental data set was decisive for obtaining a more reliable harmonic potential. The resulting force fields are presented and used, together with the ground state rotational constants, to calculate an r(z) structure. The experimental r(0), r(s), and r(m) structures have also been determined. The different results have been compared and it is concluded that the ab initio structure is a good approximation of the equilibrium structure. It is also shown that the magnetic correction is not negligible, particularly for the inertial defect. Another interesting conclusion is that the anharmonicity of the C-H stretching might be unusually small. Copyright 2001 Academic Press.