Free-jet rotational spectrum and tunneling motion of difluoromethane...xenon

The rotational spectra of three isotopologues of difluoromethane...xenon have been investigated by free-jet millimeter-wave absorption spectroscopy. Only mu(c)-type transitions have been observed, all of them evenly split due to the internal motion of Xe relative to the difluoromethane moiety. The vibrational splitting, 39.1(3) MHz, has been used to estimate the tunneling barrier, V(2) = 109 cm(-1). Information on the dissociation energy has been deduced from centrifugal distortion effects (E(B) = 1.8 kJ mol(-1)). The xenon atom lies in the sigma(v) symmetry plane of difluoromethane containing the hydrogen atoms, at an r(0) distance of 3.816 A from its center of mass (cm), and forms a Xe-cm-C r(0) angle of 118 degrees . The observed conformation is in agreement with the minimum found with a distributed polarizability model.     

Calculation of argon trimer rovibrational spectrum

Rovibrational spectra of Ar3 are computed for total angular momenta up to J=6 using row-orthonormal hyperspherical coordinates and an expansion of the wave function on hyperspherical harmonics. The sensitivity of the spectra to the two-body potential and to the three-body corrections is analyzed. First, the best available semiempirical pair potential (HFDID1) is compared with our recent ab initio two-body potential. The ab initio vibrational energies are typically 1-2 cm-1 higher than the semiempirical ones, which is related to the slightly larger dissociation energy of the semiempirical potential. Then, the Axilrod-Teller asymptotic expansion of the three-body correction is compared with our newly developed ab initio three-body potential. The difference is found smaller than 0.3 cm-1. In addition, we define approximate quantum numbers to describe the vibration and rotation of the system. The vibration is represented by a hyper-radial mode and a two-degree-of-freedom hyperangular mode, including a vibrational angular momentum defined in an Eckart frame. The rotation is described by the total angular momentum quantum number, its projection on the axis perpendicular to the molecular plane, and a hyperangular internal momentum quantum number, related to the vibrational angular momentum by a transformation between Eckart and principal-axes-of-inertia frames. These quantum numbers provide a qualitative understanding of the spectra and, in particular, of the impact of the nuclear permutational symmetry of the system (bosonic with zero nuclear spin). Rotational constants are extracted from the spectra and are shown to be accurate only for the ground hyperangular mode.     

The rotational spectrum and structure for the argon-cyclopentadienyl thallium van der Waals complex: experimental and computational studies of noncovalent bonding in an organometallic pi-complex

The rotational spectrum of a noble gas-organometallic complex was measured using a pulse molecular beam Fourier transform microwave spectrometer. Rotational transitions for the neutral argon-cyclopentadienyl thallium weakly bound complex were measured in the 4-9 GHz range. Analysis of the spectrum showed that the complex is a prolate symmetric-top rotor with C(5V) symmetry. The experimentally determined molecular parameters for Ar-C(5)H(5) (205)Tl are B=372.4479(3) MHz, D(J)=0.123(2) kHz, and D(JK)=0.45(2) kHz. For Ar-C(5)H(5) (203)Tl, B=373.3478(5) MHz, D(J)=0.113(3) kHz, and D(JK)=0.37(3) kHz. Using a pseudodiatomic model with Lennard-Jones potential yields an approximate binding energy of 339 cm(-1). The argon atom is located on the a-axis of the C(5)H(5)Tl monomer, directly opposite from the thallium metal atom. The measured separation distance between argon and the cyclopentadienyl ring is R=3.56 A. The overall size of the cluster is about 6 A, measuring from argon to thallium. Relatively small D(J) and D(JK) centrifugal distortion constants were observed for the complex, indicating that the structure of Ar-C(5)H(5)Tl is somewhat rigid. MP2 calculations were used to investigate the possible structures and binding energies of the argon-cyclopentadienyl thallium complex. Calculated, counterpoise corrected binding energies are evaluated at R=3.56 A for Ar-C(5)H(5)Tl range from 334 to 418 cm(-1). The experimental binding energy epsilon=339 cm(-1) for Ar-C(5)H(5)Tl falls within this range. The higher-level MP2/aug-cc-pVTZ-PP (thallium)/aug-cc-pVTZ(Ar, C, H) calculation with variable R yielded R(e)=3.46 A and binding energy of 535 cm(-1). Our estimated binding energy for argon-cyclopentadienyl thallium is very similar to the binding energy of argon-benzene. Calculations for the new van der Waals complexes, Ar(C(5)H(5)Tl)(2) and (C(5)H(5)Tl)(2), have been obtained, providing further information on the structures and bonding properties of previously observed cyclopentadienyl thallium polymer chains. The calculated intermolecular distance R(Tl-Cp)=3.05 A for the (CpTl)(2) chain subunit (Cp is cyclopentadienyl, C(5)H(5)) is slightly longer than the measured x-ray value R(M-Cp)(M=Tl)=2.75 A. The x-ray distance R(Tl-Tl)=5.5 A for the chain structure is almost identical to the calculated R(Tl-Tl)=5.51 A for the (C(5)H(5)Tl)(2) dimer.     

Infrared spectrum and structural assignment of the water trimer anion

The bending vibrational spectrum of the perdeutero isotopomer of the water trimer anion has been measured and compared with spectra calculated using the MP2, CCSD, and Becke3LYP electronic structure methods. Due to its low electron binding energy (approximately 150 meV), only the OD bending region of the IR spectrum of (D2O)3(-) is accessible experimentally, with electron ejection dominating at higher photon energies. The calculated spectrum of the isomer having three water molecules arranged in a chain agrees best with the experimental spectrum. In the chain isomer, the excess electron is bound to the terminal water monomer with two dangling OH groups. This is consistent with the electron binding mechanism established previously for the (H2O)n(-) (n = 2, 4-6) anions.     

Water-ketones hydrogen bonding: the rotational spectrum of cyclobutanone-water

The hydrogen-bonded complex cyclobutanone-water has been studied by Fourier-transform molecular-beam microwave spectroscopy in the frequency range of 6-18.5 GHz. The rotational spectra of ten isotopomers have been assigned and measured. Five of them have been obtained from different isotopic species (or configurations) of water (H2O, D2O, DOH, HOD, and H2 18O). The remaining five correspond to the four singly substituted 13C and to the 18O species of cyclobutanone, observed in natural abundance. For all species the inertial defect is in the range from -10.44 to -10.50 uA2, showing that the cyclobutanone frame is effectively planar and that the water molecule is coplanar to this frame. The hydrogen bond, almost linear, is formed between a water proton and one of the lone pairs of the cyclobutanone oxygen.     

Self-assembled trimer structures highlight the competitive roles of intermolecular and adsorbate-substrate interactions: PVBA trimer on Pd(111)

We observed the orientation of 4-trans-2-(pyrid-4-yl-vinyl)benzoic acid (PVBA) trimers on Pd(111) using scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV). The image showed three different types of trimers, one of which does not follow predicted dimer orientations. This type of trimer displays 10° rotations of each molecule in clockwise or counterclockwise directions. Calculations of adsorbate-substrate energy and hydrogen bonding energy revealed that the rotations are a result of competition between adsorbate-adsorbate and adsorbate-substrate interactions.     

Revealing noncovalent interactions in quantum crystallography: Taurine revisited

The charge density distribution in taurine (2‐aminoethane‐sulfonic acid) is further studied with the molecular orbital occupation number refinement scheme. The recently proposed NCIPLOT scheme (Johnson et al., J. Am. Chem. Soc. 2010, 132, 6498) is applied to visualize the noncovalent interactions from experimentally refined charge densities. Herein, we demonstrate the evolution of the reduced density gradient isosurface during the charge density refinement process. © 2012 Wiley Periodicals, Inc.     

The rotational spectrum of CN-

The rotational spectrum of the molecular negative ion CN(-) has been detected in the laboratory at high resolution. The four lowest transitions were observed in a low pressure glow discharge through C(2)N(2) and N(2). Conclusive evidence for the identification was provided by well-resolved nitrogen quadrupole hyperfine structure in the lowest rotational transition, and a measurable Doppler shift owing to ion drift in the positive column of the discharge. Three spectroscopic constants (B, D, and eQq) reproduce the observed spectrum to within one part in 10(7) or better, allowing the entire rotational spectrum to be calculated well into the far IR to within 1 km s(-1) in equivalent radial velocity. CN(-) is an excellent candidate for astronomical detection, because the CN radical is observed in many galactic molecular sources, the electron binding energy of CN(-) is large, and calculations indicate CN(-) should be detectable in IRC+10216-the carbon star where C(6)H(-) has recently been observed. The fairly high concentration of CN(-) in the discharge implies that other molecular anions containing the nitrile group may be within reach.     

The rotational spectrum of the cyclopentadienylallylnickel complex

The rotational spectrum of cyclopentadienylallylnickel, C₃H₅NiC₅H₅, has been studied using a pulsed molecular beam Fourier transform microwave spectrometer. Twelve a-type transitions were analyzed to obtain rotational and centrifugal distortion constants for the parent C₃H₅⁵⁸NiC₅H₅ complex. The measured rotational constant A = 3107.603(93) MHz is about 160.0 MHz larger than the predicted DFT value, providing evidence for possible fluxional motion in the complex. The large distortion constants, on the order of 100 kHz, provide further evidence for fluxional motion. The experimental constants B = 1302.38(22) and C = 1276.40(15) MHz are in good agreement with the DFT calculated values and confirm the η³-bonding of the allyl ligand to the Ni–C₅H₅ moiety. DFT calculations provide a V₅ barrier for internal rotation about the Ni–C₅H₅ axis of 53 cm⁻¹, with the lowest energy conformation having the central allyl c-atom eclipsed with respect to two C₅H₅ carbon atoms. Several additional rotational lines, possibly those of an exited torsional state, were observed but not assigned.     

Ion-ion proton transfer reactions of bio-ions involving noncovalent interactions: Holomyoglobin

Multiply protonated horse skeletal muscle holomyoglobin and apomyoglobin have been subjected to ion-ion proton transfer reactions with anions derived from perfluoro-1,3-dimethylcyclohexane in a quadrupole ion trap operated with helium as a bath gas at 1 mtorr. Neither the apomyoglobin nor holomyoglobin ions show any sign of fragmentation associated with charge state reduction to the 1 + charge state. This is particularly noteworthy for the holomyoglobin ions, which retain the noncovalently bound heme group. For example, no sign of heme loss is associated with charge state reduction from the 9 + charge state of holomyoglobin to the 1 + charge state despite the eight consecutive highly exothermic proton transfer reactions required to bring about this charge change. This result is consistent with calculations that show the combination of long ion lifetime and the high ion-helium collision rate relative to the ion-ion collision rate makes fragmentation unlikely for high mass ions in the ion trap environment even for noncovalently bound complexes of moderate binding strength. The ion-ion proton transfer rates for holo- and apomyoglobin ions of the same charge state also were observed to be indistinguishable, which supports the expectation that ion-ion proton transfer rates are insensitive to ion structure and are determined primarily by the attractive Coulomb field.     

Microwave spectrum and rotational isomers of epifluorohydrin

The microwave spectra of three rotational isomers of epifluorohydrin are Presented. The ground and 3 excited torsional states of one of the two gauche rotamers (III), and the ground states of the other gauche and cis rotamers (I & II), have been observed. The values of the ground state rotational constants. A, B, and C for rotamers I, II and III are 14 662.8, 3129.4, 2863.3; 10 645.8, 4050.2, 3679.1; and 14 833.1, 3210.2, 2933.9 MHz. The gaseous phase dipole components for the most abundant rotamer (III) are μ_a = 0.99 ± .02, μ_b = 2.90 ± .01 and μ_c = 0.0 ± .3, giving μ = 3.08 ± .02 D.     

The rotational spectrum and structure of HOOOH

Dihydrogen trioxide, HOOOH, which is a species with fundamental importance for understanding the chain formation ability of the oxygen atom, was detected in a supersonic jet by a Fourier transform microwave spectrometer with a pulsed discharge nozzle, together with double resonance and triple resonance techniques. Its precise molecular structure was determined from the experimentally determined rotational constants of HOOOH and its isotopomer, DOOOD. Many of the microwave and millimeter wave transitions can now be accurately predicted, which could be facilitated for remote sensing of the molecule to elucidate its roles in various chemical processes.     

Jet-cooled rotational spectrum of laser-ablated phenylalanine

The rotational spectrum of neutral phenylalanine has been recorded for the first time using laser-ablation molecular-beam Fourier transform microwave spectroscopy (LA-MB-FTMW). Two conformers stabilized by conjugative O-H···N and N-H···π hydrogen bond interactions have been conclusively identified on the basis of experimental values of rotational and (14)N nuclear quadrupole coupling constants. The nonobservation of the rotational spectra of the other low-energy conformers has been attributed to the photofragmentation that takes place in the laser ablation process. Nuclear quadrupole coupling interactions have been used directly to determine the orientation of the amino group and to identify and experimentally characterize the N-H···π interactions.     

The rotational spectrum of propynyl isocyanide

Propynyl isocyanide, CH₃C₂NC, has been prepared by vacuum pyrolysis of pentacarbonyl-(1,2-dichloropropenyl isocyanide) chromium, (CO)₅Cr–CN–C(Cl)=C(Cl)CH₃, and its ground state millimeter and microwave spectrum has been observed for the first time. r_s structural parameters of this molecule with a C_3v symmetry could be obtained from the rotational constants of several isotopomers: r_(C1–C2)=1.456(2) Å, r_(C2–C3)=1.206(2) Å, r_(C3–N)= 1.316(2) Å, r_(N–C4)= 1.175(2) Å, r_(H–C1)= 1.090(1) Å, >HCC=110.7(4)°. The nitrogen quadrupole coupling constant has been determined to be 878(2) kHz and measurements of the Stark effect allowed to obtain an electric dipole moment of 4.19(3) Debye. The results fit well into a series of related compounds and are in good agreement with data from ab initio calculations.     

The far-infrared rotational spectrum of the CF radical

The rotational spectrum of CF in its ground electronic state was studied around 1000 GHz, using a tunable far-infrared source. Seven transitions were observed originating from the ²Π_1/2 and ²Π_3/2 substates. The hyperfine and Λ-type splittings were resolved. The results were combined with gas-phase electron resonance and infrared diode laser spectra to determine all pertinent molecular constants.     

Sizing the Ubbelohde effect: the rotational spectrum of a tert-butylalcohol dimer

The H → D isotopic substitution of the hydroxylic hydrogen participating in the O-H···O hydrogen bond in the tert-butylalcohol dimer produces an increase of the B and C rotational constants, according to the shrinkage of the OO distance of about 7 m?, underlying and sizing the associated Ubbelohde effect.     

Redox modulation of benzene triimides and diimides via noncovalent interactions

[reaction: see text] Mellitic triimides undergo three sequential one-electron reduction processes whose potentials are significantly lowered in the presence of alkyl thioureas. The two sequential reductions of benzene diimides are similarly stabilized. Calculation of the relative free energy change between the different electronic states of the imide acceptors and their corresponding alkyl thiourea complexes indicates dramatic increases in hydrogen bond strength with increasing acceptor charge density.     

Molecular surface electrostatic potentials in the analysis of non-hydrogen-bonding noncovalent interactions

Electrostatic potentials computed on molecular surfaces are used to analyse some noncovalent interactions that are not in the category of hydrogen bonding, e.g. “halogen bonding”. The systems examined include halogenated methanes, substituted benzenes,s-tetrazine and l,3-bisphenylurea. The data were obtained byab initio SCF calculations.     

The electron spin resonance spectrum of the lithium trimer in hydrocarbon matrices

The lithium metal trimer, Li₃, has been prepared in adamantane using a rotating cryostat and its electron spin resonance spectrum recorded from 4 to 298 K. It has three magnetically equivalent Li nuclei with parameters a₇(3) = 33.1 G and g = 2.001. Consequently a change of matrix from argon to adamantane has no effect on the spin distribution and structure of Li₃ which appears to be a pseudorotating dynamic Jahn-Teller molecule at all temperatures. In adamantane it is stable for several hours even at room temperature.