L-type Doubling Constants Research Articles

Structures of the linear silicon carbides SiC4 and SiC6: Isotopic substitution and Ab Initio theory

The structures of two linear silicon carbides, SiC4 and SiC6, have been determined by a combination of isotopic substitution and large-scale coupled-cluster ab initio calculations, following detection of all of the singly substituted isotopic species in a supersonic molecular beam with a Fourier transform microwave spectrometer. Rotational constants obtained by least-squares fitting transition frequencies were used to derive experimental structures; except for those nearest the center of mass, individual bond lengths for both chains have an error of less than 0.008 Å. Accurate equilibrium structures were derived by converting the experimental rotational constants to equilibrium constants using the vibration–rotation coupling constants from coupled-cluster calculations, including connected triple substitutions. Equilibrium dipole moments and harmonic vibrational frequencies were also calculated for both chains. On the basis of the calculated vibration–rotation and l-type doubling constants, weak rotational satellites from a low-lying vibrational state of SiC4 were assigned to v6, a bending mode calculated to lie about 205 cm−1 above the ground state. A recommended ab initio equilibrium structure for SiC8 has also been established.

HC3P: results of coupled cluster calculations

On the basis of CCSD(T) calculations, an accurate equilibrium geometry (uncertainties in bond lengths: ca. 0.0005Å) has been established for linear HC3P. The CP equilibrium bond length is 1.5515Å and thus slightly longer than in HCP. Harmonic vibrational wavenumbers, vibration–rotation coupling constants and l-type doubling constants are reported. Accurate ground-state rotational constants are predicted for 13C substituted species.

NC 3NC: a combined millimetre-wave spectroscopic and ab initio investigation

The millimetre-wave spectrum of 2-isocyano-3-propynenitrile, NC 3NC, was observed in the vibrational ground state (0000) and in the excited bending states ( v 6 v 7 v 8 v 9)=(1000), (0100), (0010), and (0001). Pure rotational transitions J+1← J were recorded in the ranges 40–120 and 240–315 GHz. The analysis of the spectra, aided greatly by accompanying ab initio calculations, yielded precisely determined rotational, centrifugal distortion, and l-type doubling constants. They are in excellent agreement with the results of coupled-cluster calculations. Wavenumbers and infrared (IR) intensities of various stretching vibrational transitions were calculated variationally. Good agreement with results from high-resolution IR spectroscopy is observed for the highest fundamentals ν 1– ν 3, with deviations amounting to 15.4 cm −1 ( ν 1), 1.2 cm −1 ( ν 2), and 7.0 cm −1 ( ν 3).

The millimeter-wave spectrum of AlNC: chemical trends in metal isocyanide molecules

The millimeter-wave spectrum of AlNC (X 1Σ +) has been recorded using direct absorption techniques in the range 131–536 GHz, including data for several quanta of the bending vibration and the 13C,isotopomer. The species was created by the reaction of aluminum vapor and trimethylsilyl cyanide. Twenty rotational transitions were recorded for the main isotopomer and nine for AlN 13C, as well as lines for the (01 10), (02 0o) states. Rotational and l-type doubling constants have been determined for this species, as well as bond lengths. These measurements indicate that AlNC is a linear and fairly rigid molecule, as well as being the favored isomer, in agreement with theory.

Anharmonic constants for benzene

The anharmonic constants x pq , g pq are recomputed for C 6H 6, using a previously obtained self consistent field quartic forcefield. Particular care has been taken to obtain correct values for the anharmonic constants when p and q refer to degenerate modes t and t′, by treating the molecule as an asymmetric top. The Pliva formulae have been verified for x tt , g tt , x tt′ and g tt′ . Values for the vibrational l-type doubling constants r tt′ , s tt′ have also been determined. The importance of Fermi-I and Fermi-II resonances has also been examined.

Investigation of a vibration-rotation interaction in DCO + by millimeter wave spectroscopy

The rotational spectrum of DCO + in the 01 11, 03 10 and 04 00 vibrational excited states has been studied to derive the rotational, centrifugal distortion and l-type doubling constants. It has been found that there is a Coriolis-type interaction involving the 01 11 and 10 00 states, thus ruling out a previous hypothesis suggesting the existence of a Fermi resonance between the 10 00 and 04 00 states.

Hot Band Spectroscopy of Acetylene after Intermolecular Vibrational Energy Transfer from Ethylene

A rotationally cooled hot band spectrum of acetylene in the 3300-cm−1 region has been recorded using an IR-IR double resonance technique on an argon-seeded supersonic jet containing two different molecular species, ethylene and acetylene. After excitation of ethylene by a cw CO2 laser, collisional energy transfer from ethylene to acetylene populates the low-lying modes (ν4, ν5, 2ν4, and 2ν5) in acetylene. Several hot bands starting from these vibrationally excited levels have been observed by applying an FCL laser as a probe. A high-resolution spectrum has been recorded from 3306 to 3268 cm−1 and from 3258 to 3256 cm−1, including six previously unresolved Q branches assigned to six different Π-Π and Δ-Δ hot band transitions. In total, more than 50 new lines have been assigned. Effective l-type doubling constants have been determined for the upper states of the hot bands. Separate linear least-squares fits have been applied to the transition wavenumbers for each sub-Q-branch, yielding rotational constants in good agreement with literature values, where available, derived from the P and R branches.

Microwave spectroscopy of fluorocyanobutadiyne (F-CC-CC-CN)

The microwave spectra of fluorocyanobutadiyne, FC 5N, in the ground, v 11 (skeletal bending) and 2 v 11 states were observed with a source-modulated spectrometer. The FC 5N molecule was generated in a free space absorption cell by a dc glow discharge in pentafluorobenzonitrile, C 6F 5CN. The rotational and centrifugal distortion constants for the ground state were determined to be 755.398325(41) MHz and 8.3458(11) Hz, respectively. The l-type doubling constant for the v 11 state, 0.508887(48) MHz, was consistent with the value calculated from the harmonic force field.

The high-resolution infrared spectrum of cyanobutadiyne, HC5N: The ν7 band centered at 642 cm

The ν7 infrared band of cyanobutadiyne (HC5N) was recorded for the first time with high resolution by FTIR spectroscopy using a sample produced by gas electric discharge. The fundamental band and the hot bands arising from the ν11 = and 2 states are identified. The molecular parameters, including the rotational and vibrational ℓ-type doubling constants, have been determined accurately by analyzing the spectra together with the available microwave and millimetre wave data. The intensity perturbation caused by the ℓ-type resonance in the (ν7,ν11) = (1,1) state has been clearly observed and discussed.

The High-Resolution Spectra of the ν 11 Band of Triacetylene near 622 cm −1: Revised Assignments for Hot Bands

The rovibrational spectra of triacetylene (C 6H 2) were recorded for the ν 11 band in high resolution by FTIR spectroscopy. The assignments for the hot-band system ( v 11, v 13) = (1, 1) ← (0, 1) have been fully revised, and reliable molecular parameters have been determined including the rotational and vibrational l-type doubling constants. The assignments have been confirmed by a simulation of the absorption profile in the Q-branch region. The intensity perturbation caused by the l-type resonance in the ( v 11, v 13) = (1, 1) state has been observed and discussed.

The Microwave Spectra of Fluoroiodoacetylene (ICCF)

The microwave spectra of fluoroiodoacetylene (ICCF) in the ground and the vibrationally excited states were observed with a source-modulated spectrometer. The ICCF molecule was generated in a free space absorption cell by a dc glow discharge in pentafluoroiodobenzene (C6F5I). Rotational transitions of ICCF in the range of J = 27-26 to 93-92 were measured not only in the ground state but also in the first vibrationally excited states ν3 (IC stretch), ν4 (CCF bend), and ν5 (ICC bend). The rotational and centrifugal distortion constants obtained for the ground state are 1200.590266(77) MHz and 50.5314(54) Hz, respectively, where the uncertainties correspond to one standard deviation. The l-type doubling constants obtained are 0.312099(43) MHz for the ν4 state and 0.66415(12) MHz for the ν5 state, which are consistent with the values calculated from the harmonic force field. The iodine quadrupole coupling constant in the ground state (−2417(13) MHz) is in good agreement with that in the ν3 state (−2416.1(99) MHz) within one standard error. Moreover, we discuss the π character and the ionic character of the IC bond from the iodine coupling constant.

Millimeter and submillimeter wave spectroscopy of HNC and DNC in the vibrationally excited states

The rotational transitions of hydrogen isocyanide (HNC) and deuterium isocyanide (DNC) in the vibrationally excited states as well as in the ground states were observed in the millimeter and submillimeter wave region. These compounds were generated in a dc glow discharge plasma containing hydrogen (or deuterium), nitrogen, and carbon atoms. The stretching vibrational modes, ν1 and ν3 states, were selectively excited in the discharge plasma; on the other hand, the bending mode ν2 state was thermally populated at the cell temperature. The precise rotational, centrifugal distortion and l-type doubling constants were obtained for all of the first vibrationally excited states as well as the ground states. The experimental equilibrium rotational constants Be are 45 496.7769(45) and 38 207.7217(105) MHz for HNC and DNC, respectively, where uncertainties correspond to one standard deviation. The equilibrium internuclear distances are also determined to be re (H–N)=0.996 0643(29) Å and re (N≡C)=1.168 3506(16) Å.

The bending frequency δNS of dinitrogen sulfide (N2S): A theoretical analysis demonstrating the importance of Coriolis coupling terms

There exists a discrepancy between theory and experiment for the N2S bending vibrational frequency. Ab initio molecular electronic structure theory has been used in order to investigate the electronic ground state of the dinitrogen sulfide (N2S) molecule. Self-consistent field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD) wave functions were employed using the three basis sets, the double zeta plus polarization (DZP), triple zeta plus double polarization (TZ2P), and TZ2P with one set of higher angular momentum polarization functions (TZ2Pf). For CCSD wave functions the connected triple excitations were included through perturbation theory [CCSD(T)]. Using the CCSD(T) method with the larger basis sets, experimental physical properties including bond lengths, rotational constant, centrifugal distortion constant, l-type doubling constant, vibrational frequencies, and isotopic shifts for the N–N stretching frequency have been reproduced with quantitative accuracy. This analysis proves the essential correctness of an earlier theoretical prediction of the N2S harmonic bending vibrational frequency.

The Equilibrium Geometry of HNCCN+

By combining experimental ground-state rotational constants and ab initio vibration-rotation coupling constants, an accurate equilibrium geometry has been determined for HNCCN+ : re(NH) = 1.0126(5) Å, R1e(NC) = 1.1404(5) Å, R2e(CC) = 1.3731(5) Å. and R3e(CN) = 1.1625(5) Å. Estimated error bars in units of the last digit are given in parentheses. Predictions are made for the rotational, centrifugal distortion, and ℓ-type doubling constants of various isotopomers.

Microwave spectroscopy of ClCCF and BrCCF in the vibrationally excited states

The microwave spectra of chlorofluoroacetylene (ClCCF) and bromofluoroacetylene (BrCCF) in the vibrationally excited states were observed with a source-modulated spectrometer. These compounds were generated in a free space absorption cell by a dc glow discharge in chloropentafluorobenzene (C 6F 5Cl) or bromopentafluorobenzene (C 6F 5Br). The rotational, centrifugal distortion and l-type doubling constants were obtained for the ν 5 states in ClCCF, and for the ν 4 and ν 5 states in BrCCF. The bromine quadrupole coupling constants of BrCCF in the excited states are well consistent with those in the ground state determined by Andolfatto et al. ( Z. Naturforsch. A 37, 1449–1453 (1982)) with the Fourier transform microwave spectroscopy.

A systematic study of molecular vibrational anharmonicity and vibration-rotation interaction by self-consistent-field higher-derivative methods. Linear polyatomic molecules

The inclusion of the anharmonicity of molecular vibrations is an important aspect of the goal of making highly accurate theoreticalpredictions of the spectroscopic properties of molecules. Recently developed analytic third derivative methods for selfconsistent-field(SCF) wavefunctions have made it possible to determine the complete cubic and quartic force fields of polyatomic molecules, thus allowing the treatment of anharmonic effects. Here we continue our systematic evaluation of the performance of such theoretical methods by studying several linear molecules which are well characterized experimentally, viz., HCN, DCN, CO 2, N 2O, OCS, C 2H 2, and C 2D 2. A number of anharmonic molecular properties have been determined, including vibration-rotation interaction constants, vibrational anharmonic constants, fundamental vibrational frequencies, sextic centrifugal distortion constants, rotational constants which include zero-point vibrational corrections, and vibrational and rotational l-type doubling constants. These anharmonic molecular constants are not as well converged with respect to basis set enlargement as those which were previously determined for asymmetric top molecules, apparently because all the molecules considered here contain multiple bonds. However, the reported anharmonic constants at the SCF level of theory are still in reasonably good agreement with the corresponding experimental constants. Significant improvements in accuracy are achieved by incorporating electron correlation at the configuration interaction singles and doubles (CISD) level of theory. Standard spectroscopic perturbationtheory methods are used in this study, which are directly and immediately applicable to larger molecular systems than those studied here.

Infrared diode laser spectroscopy of the ν3 fundamental of the CD3 radical

The infrared absorption spectrum of the ν3 fundamental band of the CD3 radical has been detected by diode laser absorption spectroscopy. The CD3 radical was produced by excimer laser photolysis of CD3I at 248 nm or (CD3)2CO at 193 nm. Molecular parameters of the v3=1 vibrational state were determined from a least-squares fit to 62 rotation–vibration transitions. In this fit, molecular parameters describing the ground state were constrained to those obtained from previous spectroscopic studies of the ν2 parallel IR band [J. M. Frye, T. J. Sears, and D. Leitner, J. Chem. Phys. 88, 5300 (1988)]. The molecular parameters determined in the present work are the band origin ν0=2381.088 60(84), B′=4.758 737(40), C′=2.373 297(34), (ζC)3=0.476 278(72), q3=0.003 76(59), D′N =0.000 187 9(5), DNK =−0.000 341 0(12), D′K =0.000 143 7(8), ηN =−0.000 005 5(36), η′K =0.000 060(35), and qN =0.000 063(17), all in cm−1 with one standard deviation in parentheses. The derived molecular parameters were compared with those for the CH3 radical v3=1 level determined previously [T. Amano, P. Bernath, C. Yamada, Y. Endo, and E. Hirota, J. Chem. Phys. 77, 5284 (1982)]. The molecular parameters of the v3=1 state of the CD3 and CH3 radicals follow the expected isotopic relationships. We have also found that the determined molecular parameters reasonably satisfy the approximate planarity relationships [J. K. G. Watson, J. Mol. Spectrsoc. 65, 123 (1977)] and the sign of the l-type doubling constant is consistent with a planar equilibrium structure.

Microwave spectrum, molecular structure, and force field of HBO

The HBO molecule was generated as a transient intermediate in the reaction of diborane with oxygen induced by a dc discharge. The J = 3 ← 2 transition was observed for H10BO and was combined with the spectra already reported to derive the rotational, centrifugal distortion, and nuclear quadrupole coupling constants for this species. The rotational and centrifugal distortion constants were determined also for D11BO, D10BO, H11B18O, and H10B18O. The measurements were extended to rotational transitions of H11BO and H10BO in the ν1, ν2, 2ν2, and ν3 states and to those of D11BO and D10BO in the ν2, 2ν2, and ν3 states. The vibrational frequencies and the rotational, vibration-rotation, l-type doubling, and centrifugal distortion constants thus determined for the six isotopic species were simultaneously analyzed to derive the equilibrium molecular structure and the harmonic and third-order anharmonic potential constants of the HBO molecule. The equilibrium bond lengths thus obtained are re(HB) = 1.166 67(41) Å and re(BO) = 1.200 68(10) Å, and the substituted, average, and effective structures were also calculated for comparison.

Microwave spectroscopy of FCCCN in the ground and excited vibrational states

Microwave spectrum of fluorocyanoacetylene (FCCCN) produced by a glow discharge in pentafluorobenzonitrile (C 6F 5CN) was observed using a source-modulated spectrometer with a free-space absorption cell. Rotational transitions of FCCCN in the range of J = 9-8 to 20-19 were measured with an accuracy of 30 kHz for the ground state as well as for the vibrationally excited states, ν 5 (<CCN bend), ν 6 (<FCC bend), ν 7 (skeletal bend), 2 ν 6, ν 5 + ν 7, ν 6 + ν 7, 2 ν 7, 3 ν 7, and 4 ν 7. The rotational and centrifugal distortion constants obtained for the ground state are 2070.9571 ± 0.0006 MHz and 90.32 ± 0.95 Hz, respectively, where the uncertainties correspond to one standard deviation. The observed l-type doubling constants for the ν 5 (0.7718 ± 0.0002 MHz), ν 6 (0.9017 ± 0.0002 MHz), and ν 7 (2.0264 ± 0.0003 MHz) states agree with the values calculated from the harmonic force field. Anomalous behavior of the centrifugal distortion constants observed for some vibrational states was interpreted as due to the l-type resonance between sublevels with different l values. The unperturbed centrifugal distortion and l-type doubling constants and the intervals between sublevels were determined. Rotational transitions in the Σ + sublevel of the 2 ν 6 as well as the ν 5 + ν 7 state are perturbed by the Fermi resonance with the ν 4 (CC stretch) vibrational state.

Rotational spectra of DCCCN in some excited vibrational states

We have observed and analyzed the millimeter-wave spectrum of deuterated cyanoacetylene (DCCCN) in 12 excited vibrational states. The Hamiltonian of K. M. T. Yamada, F. W. Birss, and M. R. Aliev ( J. Mol. Spectrosc. 112, 347–356 (1985)) has allowed the determination of accurate rotational and l-type doubling constants for the following isolated states: ( v 4, v 5, v 6, v 7) = (1001), (0001), (0002), (0003), (0010), (0100), (0101), (0011), and 0012). The anharmonic resonance between the (1000), (0020), and (0004) states has been observed and analyzed, yielding the unperturbed rotational constants and the Fermi interaction constant.