Inertial Defect Research Articles

Rotational analysis of bands in the high-resolution infrared spectra of cis,cis- and trans,trans-1,4-difluorobutadiene-2-d1

Pure samples of cis,cis- and trans,trans-1,4-difluorobutadiene-2-d1 have been synthesized, and high-resolution (0.0015cm−1) infrared spectra have been recorded for these nonpolar molecules in the gas phase. For the cis,cis isomer, the rotational structure in two C-type bands at 775 and 666cm−1 and one A-type band at 866cm−1 has been analyzed to yield a combined set of 2020 ground state combination differences (GSCDs). Ground state rotational constants fit to these GSCDs are A0=0.4195790(4), B0=0.0536508(8), and C0=0.0475802(9)cm−1. For the trans,trans isomer, three C-type bands at 856, 839, and 709cm−1 have been investigated to give a combined set of 1624 GSCDs. Resulting ground state rotational constants for this isomer are A0=0.9390117(8), B0=0.0389225(4), and C0=0.0373778(3)cm−1. Small inertial defects confirm the planarity of both isomers in the ground state. Upper state rotational constants have been determined for most of the transitions. The ground state rotational constants for the two isotopologues will contribute to the data set needed for determining semiexperimental equilibrium structures for the nonpolar isomers of 1,4-difluorobutadiene.

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·Å(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) Å, R(C-S) = 1.66(2) Å, and angle (N-O-H) = 105(4)° for the ground state structure.

The high resolution FTIR spectrum of the ν2 band of formaldehyde isotopomer H 213CO

The Fourier transform infrared (FTIR) absorption spectrum of the ν 2 fundamental band of the formaldehyde isotopomer H 2 13CO was recorded at an unapodized resolution of 0.0063 cm −1 in the 1630–1780 cm −1 region. Upper state ( ν 2 = 1) rovibrational constants inclusive of three rotational, five quartic, and six sextic centrifugal distortion constants were accurately determined by assigning and fitting 447 unperturbed infrared transitions with a rms deviation of 0.00056 cm −1 using Watson’s A-reduced Hamiltonian in the I r representation. Analysis of new transitions measured in this work yielded more higher-order upper state constants with greater accuracy than previously reported. The band center of the A-type ν 2 band was found to be 1707.980943 ± 0.000058 cm −1 while the calculated inertial defect Δ 2 of the H 2 13CO molecule was 0.09581 ± 0.00004 μÅ 2.

The pure rotational spectrum of HPS ($\tilde{\rm X}^1 {\rm A}^{\prime}$X̃1A′): Chemical bonding in second-row elements

The pure rotational spectrum of HPS, as well as its (34)S and D isotopologues, has been recorded at microwave, millimeter, and submillimeter wavelengths, the first observation of this molecule in the gas phase. The data were obtained using a combination of millimeter direct absorption, Fourier transform microwave (FTMW), and microwave-microwave double-resonance techniques, which cover the total frequency range from 15 to 419 GHz. Quantum chemical calculations at the B3LYP and CCSD(T) levels were also performed to aid in spectral identification. HPS was created in the direct absorption experiment from a mixture of elemental phosphorus, H(2)S, and Ar carrier gas; DPS was produced by adding D(2). In the FTMW study, these species were generated in a pulsed discharge nozzle from PH(3) and H(2)S or D(2)S, diluted in neon. The spectra recorded for HPS and its isotopologues exhibit clear asymmetric top patterns indicating bent structures; phosphorus hyperfine splittings were also observed in HPS, but not DPS. Analysis of the data yielded rotation, centrifugal distortion, and phosphorus nuclear spin-rotation parameters for the individual species. The r(m) ((1)) structure for HPS, calculated from the rotational constants, is r(H-P) = 1.438(1) Å, r(P-S) = 1.9320(1) Å, and θ(H-P-S) = 101.85(9)°. Empirically correcting for zero-point vibrational effects yields the geometry r(e)(H-P) = 1.4321(2) Å, r(e)(P-S) = 1.9287(1) Å, and θ(e)(H-P-S) = 101.78(1)°, in close agreement with the r(m) ((1)) structure. A small inertial defect was found for HPS indicating a relatively rigid molecule. Based on these data, the bonding in this species is best represented as H-P=S, similar to the first-row analog HNO, as well as HNS and HPO. Therefore, substitution of phosphorus and sulfur for nitrogen and oxygen does not result in a dramatic structural change.

The ν12 band of C 2D 4

The Fourier transform infrared (FTIR) absorption spectrum of the ν 12 fundamental band of ethylene- d 4 (C 2D 4) was recorded in the 1017–1137 cm −1 region with an unapodized resolution of 0.0063 cm −1. Upper state ( v 12 = 1) rovibrational constants consisting of three rotational and five quartic constants were improved by assigning and fitting 2103 infrared transitions using Watson’s A-reduced Hamiltonian in the I r representation. The band centre of the A-type ν 12 band is found to be 1076.98480 ± 0.00002 cm −1. The present analysis covering a wider wavenumber range and higher J and K c values yielded upper state constants including the band centre which are more accurate than previously reported. The rms deviation of the upper state fit is 0.00045 cm −1. Improved ground state rovibrational constants were also determined from the fit of 1247 ground state combination differences (GSCD) from the presently-assigned infrared transitions of the ν 12 band of C 2D 4. The rms deviation of the GSCD fit is 0.00049 cm −1. In the rovibrational analysis, local frequency perturbations were not detected even at high J and K a values. The calculated inertial defect Δ 12 is 0.32551 ± 0.00001 μÅ 2. The line intensities of the individual transitions in the ν 12 band were measured and the band strength of 39.8 ± 2.0 cm −2 atm −1 was derived for the ν 12 band of C 2D 4.

Rotationally Resolved Electronic Spectroscopy of 1,4‐Benzodioxan: The Anomeric Effect in the Ground and Electronically Excited State

We measured the rotationally resolved electronic spectra of the origin and of three vibronic bands of 1,4-benzodioxan. From comparison to various ab-initio-calculated structures of 1,4-benzodioxan, the twisted C(2) symmetric 1,4-benzodioxan was shown to be responsible for all the observed spectral features. We analyzed the inertial defects in both electronic states as sensitive indicators of the non-planarity of the system. The molecule was found to be more planar in the electronic ground state than in the electronically excited singlet state. This effect can be traced back to an increased puckering of the dioxan ring, which also comprises the oxygen atoms, in the excited state. This observation is discussed in terms of natural bond orbitals.

Improved rovibrational constants for the ν12 band of C2H3D

Abstract The Fourier transform infrared absorption spectrum of the ν12 fundamental band of ethylene-d (C2H3D) was recorded at an unapodized resolution of 0.0063 cm−1 in the 1330–1475 cm−1 region. Upper state (ν12 = 1) rovibrational constants inclusive of three rotational, five quartic, and four sextic centrifugal distortion constants were improved by assigning and fitting 1444 infrared transitions using Watson’s A-reduced Hamiltonian in the Ir representation. The present analysis yielded more higher-order upper state constants than previously reported. The rms deviation of the fit is 0.00055 cm−1. Improved ground state rovibrational constants were also determined from the combined fit of 2026 ground state combination differences (GSCD) from the assigned infrared transitions of the ν12, ν3 and ν6 bands and 21 microwave frequencies of C2H3D. The rms deviation of the GSCD fit is 0.00047 cm−1. The A-type ν12 band is centered at 1400.76262 ± 0.00004 cm−1. Local frequency perturbations were not detected in the analysis. The calculated inertial defect Δ12 is 0.20809 ± 0.00003 μA2.

Microwave spectrum and structural parameters for the formamide-formic acid dimer

The rotational spectra for six isotopologues of the complex formed between formamide and formic acid have been measured using a pulsed-beam Fourier transform microwave spectrometer and analyzed to obtain rotational constants and quadrupole coupling parameters. The rotational constants and quadrupole coupling strengths obtained for H (12)COOH-H(2) (14)NCOH are A = 5889.465(2), B = 2148.7409(7), 1575.1234(6), eQq(aa) = 1.014(5), eQq(bb) = 1.99(1), and eQq(cc) = -3.00(1) MHz. Using the 15 rotational constants obtained for the H (13)COOH, HCOOD, DCOOH, and H(2) (15)NCHO isotopologues, key structural parameters were obtained from a least-squares structure fit. Hydrogen bond distances of 1.78 Å for R(O3⋯H1) and 1.79 Å for R(H4⋯O1) were obtained. The "best fit" value for the angle(C-O-H) of formic acid is significantly larger than the monomer value of 106.9° with an optimum value of 121.7(3)°. The complex is nearly planar with inertial defect Δ = -0.158 amu Å(2). The formamide proton is moved out of the molecular plane by 15(3)° for the best fit structure. Density functional theory using B3PW91, HCTH407, and TPSS as well as MP2 and CCSD calculations were performed using 6-311++G(d,p) and the results were compared to experimentally determined parameters.

High Resolution Millimeter-Wave Spectroscopy of Vinyltellurol

The millimeter-wave rotational spectrum of vinyltellurol has been recorded and assigned for the first time. To support the spectrum assignment, high level ab initio calculations have been carried out. Geometries, total electronic energies, and harmonic vibrational frequencies have been determined at the MP2 level. A small-core relativistic pseudopotential basis set (cc-pVTZ-PP) was employed to describe the tellurium atom. Two stable conformers, synperiplanar (sp) and anticlinal (ac), have been identified. The sp conformer is planar with a small negative inertia defect of -0.025 u Å(2). The ac conformer was found to be nonplanar with a C-C-Te-H dihedral angle of about 140° from sp. This conformer exhibits a large amplitude motion associated with the torsion about the C-Te bond. The barrier to internal rotation is about 1 kJ/mol, according to the theoretical calculations. For the ac conformation, a torsional potential function consisting of quartic and quadratic terms of the torsional angle has been partially determined from the observed rotational constants.

Rotationally Resolved S1−S0 Electronic Spectra of 2,6-Diaminopyridine: A Four-Fold Barrier Problem

A comparison of the electronic properties of the nitrogen-containing rings aniline, 2-aminopyridine, and 2,6-diaminopyridine (26DAP) shows that the potential energy surface of the molecule is significantly affected as more nitrogen atoms are added to the system. High resolution, rotationally resolved spectra of four vibrational bands in the S(1)-S(0) electronic transition of 26DAP were obtained in order to explain these changes. The zig-zagging inertial defects point to a double minimum excited state potential energy surface along the coupled amino group inversion vibrational mode, which becomes a four-fold well (and barrier) problem when the existence of two nearly degenerate isomers is taken into account. Assuming that the molecules are in the lower energy, opposite-side configuration, ab initio calculations were performed using the MP2/6-31G** level of theory to create a potential energy surface modeling the simultaneous antisymmetric NH(2)-inversion mode. The calculated potential energy surface shows a ground electronic state barrier to simultaneous NH(2) inversion of ~220 cm(-1), and a fit to experimental vibrational energy level spacings and relative intensities produces an excited electronic state barrier of ~400 cm(-1). The ground state barrier is less than that in aniline, but the excited state barrier is larger.

ChemInform Abstract: The Microwave Spectra of Pyrrole-3-carbonitrile and Pyrrole-2- carbonitrile.

Abstract The microwave spectra of pyrrole-3-carbonitrile, pyrrole-2-carbonitrile, and their deuterated species generated by pyrolysis of triazole[4,5- c ]pyridine, triazole[4,5- b ]pyridine, and their deuterated precursors have been observed in the frequency range from 8.0 to 40.0 GHz. We have determined the rotational and centrifugal distortion constants and the inertial defects of pyrrole-3-carbonitrile, pyrrole-2-carbonitrile, and their deuterated species in their ground vibrational states. For normal species of pyrrole-2-carbonitrile, we have extended to the determination of the constants in the excited vibrational states. The two vibrational states assigned are for the out-of-plane and the in-plane vibrational modes. The dipole moments (Debye) obtained are μ a = 4.56(27), μ b = 1.334(4), and μ total = 4.75(27). Both pyrrole-3-carbonitrile and pyrrole-2-carbonitrile are shown to be planar molecules from their inertial defects. The identification of the deuterated species allows us to clarify the pyrolysis mechanism of triazole[4,5- c ]pyridine and triazole[4,5- b ]pyridine.

ChemInform Abstract: Conformation and Internal Rotation in 2-Nitrofuran and 2- Nitrothiophene by Microwave Spectroscopy.

Abstract Results of a microwave investigation of the molecules 2-nitrofuran and 2-nitrothiophene are reported. Transitions in the ground state, torsional excited states and out-of-plane and in-plane excited states of the bending vibration of the nitro group have been assigned. The rotational parameters are consistent with a planar equilibrium configuration for both molecules. The torsional frequency has been derived from relative intensity measurements and inertial defect values. The barrier hindering internal rotation has been calculated to be 17kJ mol−1 and 19kJ mol−1 for 2-nitrofuran and 2-introthiophene, respectively.

ChemInform Abstract: Pyrolysis Jet Spectroscopy: The S1-S0 Band System of Formyl Cyanide, HCOCN, and DCOCN

The A 1A‘–X 1A’ band systems of HCOCN and DCOCN have been studied in depth using the technique of pyrolysis jet spectroscopy. Ab initio predictions of molecular structures and vibrational frequencies for the ground and excited state were made, to assist in the assignment of the vibronic structure in the laser‐induced fluorescence (LIF) spectra. A resolved emission spectrum obtained by laser population of the 81 level of the excited state was analyzed to establish the ground state vibrational frequencies. Both the LIF and resolved emission spectra are consistent with a substantial elongation of the C=O bond on excitation, and smaller changes in the other geometric parameters. The excited state inertial defect, the pattern of bands involving the antisymmetric vibrations, and the ab initio predictions all suggest that the excited state structure is only slightly nonplanar. A rotational analysis of the 910 band of the DCOCN LIF spectrum has provided the first experimental rotational constants for the ground...

The high-resolution FTIR spectrum of the ν6 band of C 2H 3D

The absorption spectrum of the ν 6 band of C 2H 3D centered near 1125.27674 cm −1 in the 1100–1250 cm −1 region was recorded with an unapodized resolution of 0.0063 cm −1 using a Fourier transform infrared (FTIR) spectrometer. A total of 947 infrared transitions of the A– B hybrid-type band were assigned and fitted to upper-state ( ν 6 = 1) rovibrational constants using a Watson’s A-reduced Hamiltonian in the I r representation up to eighth-order centrifugal distortion terms. The b-type infrared transitions of the band were analyzed for the first time. The root-mean-square deviation of the fit was 0.00062 cm −1. The ground-state rovibrational constants up to eighth-order terms were also obtained by a fit of 617 combination differences from the present infrared measurements, simultaneously with 21 microwave frequencies with a root-mean-square deviation of 0.00055 cm −1. From this work, the upper-state ( ν 6 = 1) and ground-state constants of C 2H 3D were derived with the highest accuracy, so far. The a- and b-type transitions of the hybrid ν 6 band were found to be relatively free from local frequency perturbations. The ratio of the a- to b-type vibrational dipole transition moments ( μ a / μ b ) was found to be 1.05 ± 0.10. From the ν 6 = 1 rovibrational constants obtained, the inertial defect Δ 6 was calculated to be 0.3570 ± 0.0008 μÅ 2.

A reevaluation of the assignment of the vibrational fundamentals and the rotational analysis of bands in the high-resolution infrared spectra of trans- and cis-1,3,5-hexatriene

Assignments of the vibrational fundamentals of cis- and trans-1,3,5-hexatriene are reevaluated with new infrared and Raman spectra and with quantum chemical predictions of intensities and anharmonic frequencies. The rotational structure is analyzed in the high-resolution (0.0013–0.0018 cm −1) infrared spectra of three C-type bands of the trans isomer and two C-type bands of the cis isomer. The bands for the trans isomer are at 1010.96 cm −1 ( ν 14), 900.908 cm −1 ( ν 16), and 683.46 cm −1 ( ν 17). Ground state (GS) rotational constants have been fitted to the combined ground state combination differences (GSCDs) for the three bands of the trans isomer. The bands for the cis isomer are at 907.70 cm −1 ( ν 33) and 587.89 cm −1 ( ν 35). GS rotational constants have been fitted to the combined GSCDs for the two bands of the cis isomer and compared with those obtained from microwave spectroscopy. Small inertial defects in the GSs confirm that both molecules are planar. Upper state rotational constants were fitted for all five bands.

High-resolution FTIR measurement and analysis of the ν3 band of C 2H 3D

The Fourier transform infrared (FTIR) spectrum of the ν 3 band of C 2H 3D was measured at an unapodized resolution of 0.0063 cm −1 in the 1240–1340 cm −1 region. Rovibrational constants for the upper state ( ν 3 = 1) up to five quartic and two sextic centrifugal distortion terms had been obtained by assigning and fitting a total of 1037 infrared transitions using a Watson’s A-reduced Hamiltonian in the I r representation. The root-mean-square deviation of the fit was 0.00051 cm −1. The ground state rovibrational constants were also determined by a fit of 674 combination differences together with 21 microwave frequencies from the present infrared measurements with a root-mean-square deviation of 0.00040 cm −1. The upper state ( ν 3 = 1) and ground state rovibrational constants of C 2H 3D represent the most accurate values obtained so far. The A-type ν 3 band, centred at 1288.788826 ± 0.000044 cm −1 was found to be relatively free from local frequency perturbations. From the ν 3 = 1 rovibrational constants obtained, the inertial defect Δ 3 was 0.1619724 ± 0.0000001 μÅ 2.

Microwave Spectrum, Structural Parameters, and Quadrupole Coupling for 1,2-Dihydro-1,2-azaborine

The first microwave spectrum for 1,2-dihydro-1,2-azaborine has been measured in the frequency range 7-18 GHz, providing accurate rotational constants and nitrogen and boron quadrupole coupling strengths for three isotopomers, H(6)C(4)(11)B(14)N, H(6)C(4)(10)B(14)N, and H(5)DC(4)(11)B(14)N. The measured rotational constants were used to accurately determine coordinates for the substituted atoms and provide sufficient data to determine most of the important structural parameters for this molecule. The spectra were obtained using a pulsed beam Fourier transform microwave spectrometer, with sufficient resolution to allow accurate measurements of (14)N, (11)B, and (10)B nuclear quadrupole hyperfine interactions. High-level ab initio calculations provided structural parameters and quadrupole coupling strengths that are in very good agreement with measured values. The rotational constants for the parent compound are A = 5657.335(1), B = 5349.2807(5), and C = 2749.1281(4) MHz, yielding the inertial defect Delta(0) = 0.02 amu x A(2) for the ground-state structure. The observed near-zero and positive inertial defect clearly indicates that the molecular structure of 1,2-dihydro-1,2-azaborine is planar. The least-squares fit analysis to determine the azaborine ring structure yielded the experimental bond lengths and 2sigma errors R(B-N) = 1.45(3) A, R(B-C) = 1.51(1) A, and R(N-C) = 1.37(3) A for the ground-state structure. Interbond angles for the ring were also determined. An extended Townes-Dailey population analysis of the boron and nitrogen quadrupole coupling constants provided the valence p-electron occupancy p(c) = 0.3e for boron and p(c) = 1.3e for nitrogen.

Re-examination of the rotationally resolved spectra of the electronic origin and several vibronic bands of 1,3-benzodioxole

The structure and vibrations of 1,3-benzodioxole are reinterpreted using rotational constants and inertial defects obtained from high resolution electronic spectroscopy. Results are examined in light of previously published theoretical calculations and conclusions are drawn regarding vibrational assignments for transitions to the excited state. Compared to the ground state, the excited S 1 state has a significantly more nonplanar geometry (and/or is considerably less rigid), owing to an enhanced anomeric effect involving the oxygen lone pair electrons.

The ν12 band of ethylene-1-13C (13C12CH4) by high-resolution FTIR spectroscopy

Abstract The Fourier transform infrared (FTIR) spectrum of the ν12 fundamental band of ethylene-1-13C (or 13C12CH4) was recorded with an unapodized resolution of 0.0063 cm−1 in the wavenumber region of 1360–1520 cm−1. Rovibrational constants for the upper state (ν12 = 1) up to five quartic and two sextic centrifugal distortion terms were derived for the first time by assigning and fitting a total of 879 infrared transitions using a Watson’s A-reduced Hamiltonian in the Ir representation. The root-mean-square deviation of the fit was 0.00066 cm−1. The ground state rovibrational constants were also determined by a fit of 523 combination-differences from the present infrared measurements, with a rms deviation of 0.00090 cm−1. The A-type ν12 band which is centred at 1439.34607 ± 0.00004 cm−1 was found to be relatively free from local frequency perturbations. From the ν12 = 1 rovibrational constants obtained, the inertial defect Δ12 was found to be 0.242826 ± 0.000002 μA2.

Vibronic coupling in indole: II. Investigation of the 1La–1Lb interaction using rotationally resolved electronic spectroscopy

High-resolution electronic spectra of indole (C(8)H(7)N) and their detailed analysis are reported. Thirteen low-lying vibronic bands--from the electronic origin transition at 35,231.4 cm(-1) up to 1000 cm(-1) above--are recorded with rotational resolution. Besides inertial parameters and inertial defects these spectra yield detailed information, for each individual band, on the transition-dipole-moment orientations in the molecular inertial frame as well as on the reorientation of that inertial frame upon electronic excitation. The natural lifetimes of the individual vibronic states have also been determined. Strongly varying orientations of the transition-dipole-moments, unexpected positive inertial defects, and decreasing lifetimes, which are only partly related to increased excitation energy, are observed. These results are clear indications of the interaction of the two lowest electronically excited singlet states ((1)L(b) and (1)L(a)). Our experimental findings are strongly supported by, and in excellent agreement with, the theoretical description of the interaction of the two electronic states described in the preceding paper. These results provide clear evidence for strong vibronic coupling of the two electronic states (1)L(b) and (1)L(a) and for the energetic location of the (1)L(a)-state more than 1000 cm(-1) above the (1)L(b) vibrationless state.