Branch Transitions Research Articles

ChemInform Abstract: THE MICROWAVE SPECTRUM OF ACETALDEHYDE CYANOHYDRIN

Abstract The microwave spectrum of acetaldehyde cyanohydrin shows two series of μ a R branch transitions of almost equal intensity which are accompanied by similar patterns of vibrational satellites. The most probable explanation of these two series of lines consistent with the expected conformational behaviour of acetaldehyde cyanohydrin is that they belong to the two non-equivalent rotamers in which the hydroxyl group is gauche to the cyanide group.

Resonance energy transfer in LiH(A 1Σ)

Measurement of the rates of quenching of specific vibrational–rotational levels of LiH(A 1Σ) by Li(2 2S) show very strongly resonant behavior, including distinct resonance with P and R branch transitions. These are interpreted in terms of resonant energy transfer to form LiH(X 1Σ) and Li(2 2P). Semiclassical calculations confirm this mechanism.

The microwave spectrum of acetaldehyde cyanohydrin

The microwave spectrum of acetaldehyde cyanohydrin shows two series of μ a R branch transitions of almost equal intensity which are accompanied by similar patterns of vibrational satellites. The most probable explanation of these two series of lines consistent with the expected conformational behaviour of acetaldehyde cyanohydrin is that they belong to the two non-equivalent rotamers in which the hydroxyl group is gauche to the cyanide group.

ChemInform Abstract: A PRECISE STUDY OF THE ROTATIONAL SPECTRUM OF FORMALDEHYDE H212C17O AND H213C17O

Abstract The pure rotational spectra of H 2 12 C 17 O and H 2 13 C 17 O have been investigated in the frequency region between 8 and 360 GHz in the ground vibrational state. For both isotopic species the 17 O nuclear quadrupole coupling constants and spin-rotation constants have been obtained. From both Q - and R -branch transitions a set of rotational constants and several distortion constants could be derived employing Watson's formalism in A reduction. The obtained rotational constants are in Megahertz: H 2 12 C 17 O H 2 13 C 17 O A 281 965.0 (30) 281 987.3 (19) B 37 812.287(45) 36 776.790(25) C 33 214.523(31) 32 412.920(19) This study reports the first measurement of the H 2 13 C 17 O rotational spectrum.

Observation of laser driven transitions to high rydberg states of He 2

We report the excitation and detection of Rydberg states of He 2 with principal quantum numbers ≲ 25. The transitions were driven from the metastable (2sσ) a 3∑ + u He 2 state, and the excited states were detected using ionization techniqaes Q branch transitions terminating on npπ 3Π − g ( v = 0) states up to n = 25 have been identified and assigned.

Hexadecapolar U-branch transitions in the infrared fundamental band of gaseous hydrogen at 77 K

U-branch transitions corresponding to the rotational selection rule ΔJ=+4 in the collision-induced fundamental band of normal H2 at 77 K have been investigated for gas densities in the range 300–520 amagat with a 2 m absorption cell. These transitions, arising from the hexadecapolar induction mechanism in the colliding pairs of molecules, occur in the high wave number tail of the overlap and quadrupolar transitions of the band and include primarily single transitions U1(J) and double transitions Q1(J)+U0(J), J=0 and 1, at 77 K. The experimental absorption profiles were analyzed by assuming a new expression for the contribution of the total wing of the overlap and quadrupolar transitions and a dispersion type function as well as a Levine–Birnbaum function for the U transitions, and the characteristic half-width parameters δu of the dispersion function and δ′u of the Levine–Birnbaum function have been obtained. The experimental value of the binary absorption coefficient of the U1(1) transition, derived from the profile analysis, is 2.40×10−10 cm−1 amagat−2, which is in good agreement with the theoretical value.

Ground state spectroscopic constants of isothiocyanic acid, HNCS, from its microwave and millimeter wave spectra combined with far infrared data

Abstract The microwave and millimeter wave spectra of isothiocyanic acid, HNCS, in the ground vibrational state have been investigated in the frequency region 8–300 GHz. The a-type R-branch transitions have been assigned up to J = 25 and Ka = 4, and the a-type qQ1 branch transitions up to J = 45. No b-type transitions could be identified in the frequency region covered. The far infrared data reported by Krakow, Lord, and Neely [J. Mol. Spectrosc., 27, 148 (1968)] were combined with our millimeter wave data in order to determine reliable spectroscopic constants. The rotational Hamiltonian, Watson's formalism with S reduction, has been extended empirically to higher order to facilitate the fitting of the large centrifugal distortion effects. The obtained constants are: A = 1357.3 GHz; B = 5883.4627 MHz; C = 5845.6113 MHz; D J = 1.19393 kHz; D JK = −1025.37 kHz; D K = 51.57 GHz; d 1 = −13.781 Hz; d 2 = −4.59 Hz. The 14N quadrupole coupling constant has also been determined: χaa = 1.114 MHz.

Absorption spectrum of deuterated hydrazoic acid, D14N3 in the microwave and millimeterwave regiont

Thirty b-type P, Q and R branch rotational transitions and nineteen a-type Q and R branch transitions have been detected and measured for the slightly asymmetric rotor molecule D14N3 in the frequency range from 8 to 340 GHz. The information obtained from the analysis of the observed frequencies are greatly improved values of the K-dependent parameters AO = 344746.589(64) MHz, and DK = 92.242(33) MHz, refined values for the rotational constants BO and CO, a complete set of quartic centrifugal distortion constants and several sextic distortion constants. Both a and b components of the electric dipole moment were determined giving a total dipole moment of μ = 1.76(5) debye.

ChemInform Abstract: MICROWAVE SPECTRUM AND CONFORMATION OF ISOPROPYL FLUOROFORMATE

Abstract The microwave spectrum of isopropyl fluoroformate is characterized by intense a -type R -branch transitions from one conformational species. The rotational constants of the ground state, A 0 = 4967.0(8) MHz, B 0 = 1704.69(2) MHz, C 0 = 1468.86(1) MHz and κ = −0.8651(2) are consistent with a τ 1 (OCOC) = 0°, τ 2 (COCH) ~35° structure. This structure can be viewed as a combination of the two conformational species found in ethyl fluoroformate. Two vibrational satellites having rotational constants A 0 = 4963(5) MHz, B 0 = 1694.11(7) MHz. C 0 = 1471.43(4) MHz and A 0 =4998(6) MHz, B 0 = 1705.21(7) MHz, C 0 = 1471.10(4) MHz have been assigned.

Microwave spectrum and rotational isomers of ethyl fluoroformate

The microwave spectrum of ethyl fluoroformate displays strong a-type R branch transitions from two rotameric forms. One species (extended form) has rotational constants A 0 = 9191.3(9) MHz, B 0 = 2112.61(1) MHz, C 0 = 1756.73(1) MHz which are consistent with a syn-anti (τ 1(OCOC) = 0°, r 2(cocc) = 180°) planar heavy atom structure. The second species (compact form) has rotational constants A 0 = 7760(3) MHz, B 0 = 2388.38(4) MHz, C 0 = 2102.47(3) MHz which are consistent with a syn-gauche (τ 1(ococ) = 0°, τ 2(cocc) ~ 90°) structure. The two conformational forms have approximately equal energy (0 ± 40 cm −1). Four vibrational satellites of the extended species have been analyzed yielding a torsional frequency around the O-ethyl bond of 70(10) cm −1. Three vibrational satellites attributed to the O-ethyl torsion of the compact species have been analyzed yielding a vibrational frequency of 90(10) cm −1. Approximate Fourier coefficients of a three term potential function for internal rotation about the O-ethyl bond have been determined. Vibrational satellites attributed to the first excited states of the O-ester torsion have been analyzed for both conformers. The torsional vibrational frequency around the O-ester bond is 110(15) cm −1 for the extended conformers and 120(20) cm −1 for the compact.

Ground state spectroscopic constants and electric dipole moment of D14N12C32S from its microwave and millimetre wave spectra

The microwave and millimetre wave spectra of D14N12C32S in its ground vibrational state have been investigated in the frequency region 8–210 GHz. A large number of a-type Q branch and b-type P and R branch transitions up to J = 62 have been assigned and measured for the first time. Their frequencies have been combined with those of the a-type R branches up to K = 5 in an analysis for rotational, centrifugal distortion, and 14N quadrupole coupling constants. Constants obtained, in Watson's S reduction formalism, are: rotational constants in megahertz: A0 = 705 511(370); B0 = 5500.4391(15); C0 = 5445.2252(15); quartic centrifugal distortion constants in kilohertz: Dj = 1.0926(25); Djk = 1333.4(12); d1 = 3.455(5) × 10−2d2 = −7.8(14) × 10−3; 14N quadrupole coupling constants in megahertz: χaa = 1.19(6), χbb−χcc <0.05. The b-component of the dipole moment, μb, has been measured using the Stark effect as 1.08(15) D; this, when combined with μa = 1.67(3) D gives a total dipole moment of 1.99(15) D, essentially parallel to the HN bond.

Observation ofU-branch transitions in the infrared fundamental band of solid hydrogen

$U$-branch transitions corresponding to 5\ensuremath{\leftarrow}1 and 4\ensuremath{\leftarrow}0 in the rotational quantum number $J$ of the ${\mathrm{H}}_{2}$ molecule have been clearly observed in the infrared fundamental band with a 3.6-cm absorbing path of the solid at \ensuremath{\sim} 10 K. These transitions arise because of the hexadecapolar contribution to the intermolecular interaction, and this work represents the first observation of such absorption in solid ${\mathrm{H}}_{2}$. A measurement of the absorption intensity was made for the case of high-purity parahydrogen.

Far infrared spectra of ring compounds. XV. The far infrared spectrum of 7-oxabicyclo-[4.1.0]-hept-3-ene

The far infrared spectrum of 7-oxabicyclo-[4.1.0]-hept-3-ene has been studied using the Jarrell–Ash 78-900 spectrophotometer at MIT. A series of nine Q branch transitions exhibiting slight negative anharmonicity has been observed beginning at 121.9 cm−1. The fact that the rocking of the oxirane ring with respect to the six-membered ring has a frequency ∼3 times that of the ring-puckering leads to near coincidence between pairs of levels and this is reflected in some irregularity in the positions of the 2→3, 3→4, and 4→5 ring-puckering transitions. In contrast to analogs of bicyclo-[3.1.0]-hexane, where the anharmonicity is substantial and the effect of the quartic term in the potential is quite noticeable, it is possible to account for the data for 7-oxabicyclo-[4.1.0]-hept-3-ene with a harmonic potential with a small cubic perturbation. H=ν0/2(P2+Q2) +CQ3, where P=−id/dQ, Q = dimensionless ring-puckering coordinate, ν0=123.7 cm−1, C=5.49 cm−1.

The microwave Q branch of the distortion moment spectrum of 13CH4

The distortion moment microwave spectrum of 13CH4 has been studied with a conventional Stark modulated instrument having a sensitivity of ∼1×10−11 cm−1. Six pure rotational Q branch transitions have been observed between 8 and 20 GHz. With the assumption that the three octic tensor distortion constants L4T, L6T, and L8T have the same values as in 12CH4, values have been obtained for the one quartic and two sextic tensor distortion constants. These are (in Hz) DT= (132 983.32±0.48); H4T=−(17.0026±0.0018); H6T= (11.1869±0.0011). The errors given here are the standard deviations obtained from the least squares analysis. The justification for the assumption is presented, and estimates are given for the absolute errors in DT, H4T, and H6T, as well as for the isotopic shifts in L4T, L6T, and L8T. The measured differences between the 13CH4 and 12CH4 values of DT, H4T, and H6T are discussed with reference to recent theoretical calculations. A method is given of using these differences to calculate the tensor energy splittings of 13CH4 from those previously published for 12CH4.

The Distortion Moment Microwave Spectrum of 12CH4 in the Ground Vibronic State

Seven Q branch transitions between 7.8 and 20 GHz have been observed in the distortion moment pure rotational spectrum of 12CH4 in its ground vibronic state. The spectrum was obtained using a conventional Stark modulated microwave spectrometer specially modified for the detection of very weak lines. From the frequencies of these seven transitions, along with the ortho–para splitting known for the (J = 2) state and two Q branch splittings measured earlier by infrared–microwave double resonance, we have determined the one quartic tensor distortion constant DT, the two sextic constants H4T and H6T, and the three octic constants L4T, L6T, and L8T. These are (in Hz): DT = (132 943.41 ± 0.71); H4T = −(16.9839 ± 0.0076); H6T = (11.0342 ± 0.0086); L4T = (20.27 ± 0.24) × 10−4; L6T = −(26.77 ± 0.35) × 10−4; L8T = −(30.0 ± 1.8) × 10−4. The errors are the standard deviations resulting from a least squares analysis. Estimates are also given of the absolute errors. These results consitute the first measurement of the octic distortion constants. The values for DT, H4T, and H6T are compared to earlier determinations. The theoretical considerations that affect the observation of these transitions are reviewed and a detailed description of the special experimental techniques used is given. The method of analysis is described. Finally, as an aid to other high resolution studies of methane and to possible astrophysical detection of the molecule, the Q branch splittings are tabulated up to J = 21.

The Microwave and Millimetre Wave Spectrum, Molecular Constants, Dipole Moment, and Structure of Isocyanic Acid, HNCO

The microwave and millimetre wave spectra of six isotopic species of isocyanic acid, HNCO, have been measured. The molecule is a very slightly asymmetric prolate rotor. Besides the a type R branches, previously reported for two species, we have assigned for the first time a type Q branches and also some intense b type P and R branch transitions. Accurate values of the rotational constants have been determined. The rotational constant A0 is particularly large, with the result that the molecule undergoes very large centrifugal distortion, and terms up to the 12th power in the angular momentum are required to fit the spectrum. 14N nuclear quadrupole hyperfine splitting has been measured and accurate values for both χaa and (χbb − χcc) have been obtained, the latter for the first time. From the Stark effect of the b type transitions we have found the b component of the dipole moment to be 1.35 ± 0.1 D, an order of magnitude larger than previously supposed, and have used it to reinterpret earlier data indicating a variation of the a component with Ka. An improved molecular structure has been obtained. Astrophysical consequences of the large b component of the dipole moment are discussed. A table of transitions of potential astrophysical interest is presented.

Magnetic fine structure of triplet exciton bands in molecular crystals: 1,2,4,5-tetrachlorobenzene

The quantum mechanics of triplet excitons in molecular crystals is used to determine the effect of a finite excitation transfer interaction between interchange equivalent molecules on the exciton dispersion and density of states and on the zero field spectral density functions for intraband branch—branch transitions. Four cases of specific interest M > D,E; M ≈ D,E; M < D,E; and M ≈ 0 arise where M is the excitation transfer integral and D and E are the molecular magnetic fine structur parameters. Of these, only M > D,E and M ≈ 0 have been considered previously. The effects are illustrated for parameters of the 1,2,4,5-tetrachlorobenzene crystal.

Ground state spectroscopic constants and dipole moment of hydrazoic acid, H14N3

Seven b-type P and R branch rotational transitions and seven a-type Q branch transitions have been measured for the slightly asymmetric rotor H14N3 in the microwave and millimeter wave regions. The information obtained from these transitions includes the rotational constant A0 = 610996.2 (6.0) MHz, the component of the dipole moment along the b-axis |μb| = 1.48 (5) debye, refined values of the rotational constants B0 and C0, and of the centrifugal distortion constants for this molecule. Combining the value of |μb| with the value of |μa| = 0.837 debye measured previously for K = 1 transitions the total dipole moment is found to be 1.70 (5) debye.

Microwave collision diameters for Q-branch transitions in formic acid

Microwave collision diameters for <i>Q</i>-branch transitions in formic acid

ChemInform Abstract: MICROWAVE SPECTRUM AND DIPOLE MOMENT OF 3,6‐DIHYDRO‐2H‐PYRAN

The microwave spectrum of 3,6‐dihydro‐2H‐pyran has been studied in the R‐band frequency range (26.5–40.0 GHz) with a Hewlett‐Packard Model 8400 Stark‐modulated microwave spectrometer. The spectrum is characterized by rather strong perpendicular transitions of a near oblate symmetric top. Transitions obeying a, b, and c dipole selection rules have been assigned and 24 low‐J R ‐branch transitions have been used to derive the rigid rotor constants for the ground vibrational state: (in MHz) A = 5085.152±0.012; B = 4849.365±0.013; C = 2712.487±0.013. Stark displacement measurements were made for several transitions and the dipole moment components determined by least squares fitting the displacements; (in Debye) |μa|=0.892±0.006; |μb|=0.507±0.010; |μc|=0.771±0.002 yielding a total dipole moment μtot = 1.283±0.005. Model calculations to reproduce the rotational constants and the observed dipole moment components indicate that the data are consistent with a twisted ring skeleton with the oxygen atom on one side ...