Microwave Spectrum,Nuclear Quadrupole Coupling Constants,and Structure of Bromodifluoromethane

The microwave spectrum of bromodifluoromethane, CHBrF₂(Halon 1201) has been studied for the first time from 7 to 40 GHz. A least-squares analysis of the observedc-type transition frequencies gave rotational and centrifugal distortion constants and components of the bromine nuclear quadrupole coupling constant tensor in the principal axes system as follows:A= 10199.7186(62) MHz,B= 2903.4150(26) MHz,C= 2360.1521(23) MHz, Δ_J= 0.660(14) kHz, Δ_JK= 2.87(11) kHz, Δ_K= 8.95 kHz, δ_J= 0.1344(24) kHz, δ_K= 3.22(15) kHz, χ_aa= 521.281(92) MHz, χ_bb− χ_cc= −38.32(9) MHz, and |χ_ac| = 187.1(26) MHz for the⁷⁹Br species;A= 10199.5567(54) MHz,B= 2876.5588(20) MHz,C= 2342.3796(18) MHz, Δ_J= 0.652(12) kHz, Δ_JK= 2.77(9) kHz, Δ_K= 8.21(61) kHz, δ_J= 0.1300(19) kHz, δ_K= 2.97(13) kHz, χ_aa= 435.61(10) MHz, χ_bb− χ_cc= −32.08(8) MHz, and |χ_ac| = 148.5(29) MHz for the⁸¹Br species. The structural parameters are calculated from all these rotational constants and the electronic properties of the carbon–bromine bond in bromodifluoromethane are evaluated from the observed nuclear quadrupole coupling constants. These molecular properties are compared with those of other related molecules.     

Microwave spectrum,centrifugal distortion constants,and quadrupole coupling constants of 3-chloropyridine

The microwave spectrum of 3-chloropyridine has been measured in the frequency region of 8.2 to 18 GHz. The rotational constants, centrifugal distortion constants, and the quadrupole coupling constants for the ³⁵Cl species are A = 5839.448 ± 0.027 MHz, B = 1604.152 ± 0.005 MHz, C = 1258.327 ± 0.004 MHz, Δ_J = 0.10 ± 0.01 KHz, Δ_JK = 0.36 ± 0.09 KHz, Δ_K = 1.18 ± 0.07 KHz, δ_J = ?0.008 ± 0.005 KHz, δ_K = 0.88 ± 0.20 KHz, χ_aa = ?70.04 ± 0.38 MHz, χ_bb = 36.68 ± 0.19 MHz. The values of rotational constants and quadrupole coupling constants for the ³⁷Cl species are A = 5840.052 ± 0.034 MHz, B = 1559.354 ± 0.01 MHz, C = 1230.739 ± 0.016 MHz, χ_aa = ?54.20 ± 1.26 MHz, χ_bb = 29.49 ± 0.48 MHz. The double bond character in the CCl bond is found to be 2%. The smaller than expected value of rotational constant A points to a “fattening” of the pyridine ring about the a-axis in contrast to 2-chloropyridine, where no such substitution effect was observed.     

Microwave spectrum,molecular structure,and nuclear quadrupole coupling constants of 3-bromopropene

The microwave spectra of the two ⁷⁹Br and ⁸¹Br isotopic species of 3-bromopropene were measured in the frequency region 14–23 GHz. The R and Q branches for a- and b-type rotational transitions of one conformer, skew, have been assigned and the rotational constants of the ground state have been determined to be A = 19 247.56 MHz, B = 1975.339 MHz, and C = 1914.761 MHz for ⁷⁹Br species, and A = 19 234.26 MHz, B = 1961.417 MHz, and C = 1901.563 MHz for ⁸¹Br species, respectively. By the analysis of the second-order perturbation treatment of the quadrupole interaction, it is found that the χ_ab element of the χ tensor primarily contributes to the anomalous hyperfine splittings. The matrix elements of products of direction cosines in terms of the symmetric top wavefunctions have been derived. The nuclear quadrupole coupling constants have been determined χ_aa = 384.2 MHz, χ_bb = ?71.9 MHz, χ_cc = ?276.3 MHz, and |χ_ab| = 358.7 MHz for ⁷⁹Br species and χ_aa = 283.2 MHz, χ_bb = ?55.6 MHz, χ_cc = ?227.6 MHz, and |χ_ab| = 296.0 MHz for ⁸¹Br species.     

The microwave spectrum and structure of the argon trifluoroacetonitrile complex

The rotational spectrum of argon trifluoroacetonitrile complex has been studied by pulsed-nozzle, Fourier transform microwave spectroscopy. Both a-type and b-type transitions have been observed. The rotational constants are A = 3053.0903(2) MHz, B = 1039.9570(2) MHz, and C = 895.5788(1) MHz. The ¹⁴N nuclear quadrupole hyperfine components of the rotational transitions have been resolved, the ¹⁴N nuclear quadrupole coupling constants are χ_aa = 1.746(1) MHz, and χ_bb − χ_cc = −6.426(2) MHz. The complex is T-shaped, with the argon atom located 3.73 Å from the center of mass of the trifluoroacetonitrile molecule.     

The Microwave Spectrum and Quadrupole Coupling Constants of cis-2-Chlorophenol

The rotational spectrum of cis-2-chlorophenol was observed from 5 to 12 GHz by molecular beam Fourier transform microwave spectrometer (MB-FTMW). The rotational and quadrupole coupling constants for the ³⁵Cl of the molecule were determined: A=2985.4479(22) MHz, B=1549.8591(3) MHz, C=1020.1915(1) MHz, χ_aa=−68.2429(83) MHz, χ₋=−0.922(20) MHz, χ_ab=−11.4(19) MHz. The quadrupole coupling constants of the chlorine nuclei of cis-2-chlorophenol were nearly equal to that of the chlorobenzenes. We concluded that the electric field gradient of the chlorine atom in cis-2-chlorophenol is similar to that of other chlorobenzenes, although the suggestion of intramolecular hydrogen bonding.     

The 14N quadrupole coupling constants and barrier to internal rotation of ethylcyanide-d5

The microwave spectrum of ethylcyanide-d₅ has been recorded from 18.0 to 40.0 GHz. Both a-type and b-type transitions were observed and assigned. Also, the R-branch assignments have been made for three excited states of the internal torsional mode and two excited states of the CN inplane bending mode as well as an excited vibrational state involving both of these motions. The barrier to internal rotation was determined to be 3.00 ± 0.15 kcal/mole from the E, A splittings of the third excited state. The quadrupole coupling constants of the¹⁴N nucleus were found to have values of ?3.213, 1.168, and 2.045 MHz for χ_aa, χ_bb, and χ_cc, respectively. These results are compared to those previously obtained on the corresponding hydrogen compound.     

Torsional analysis of 2-butynol

The microwave spectrum of 2-butynol, CH₃CCCH₂OH, has been investigated in the frequency range of 6-26.5 GHz. The spectra of the A and E torsional states were observed using Fourier transform microwave spectroscopy. Due to the presence of the cylindrically symmetric CC “spacer” between the methyl group and the rest of the molecule, the barrier to internal rotation, V₃, is only 7 cm⁻¹. One conformer of 2-butynol was observed and assigned. The spectrum was analyzed with the ρ-axis method using a very flexible Hamiltonian which gives a fit an order of magnitude better than that obtained with more standard code for internal rotation. The spectroscopic constants are: 23744.(18), 2093.429(1), 1966.358(1), and −400.34(2) MHz for A, B, C, and D_ab; 0.48(1) kHz, −30.3(4) kHz, and 4.5(5) MHz for Δ_J, Δ_JK, and Δ_K; and 0.1406(6), 6.93(9) cm⁻¹, −33.4(7) kHz, and 192.0(9) GHz for ρ, V₃, L_V, and F, respectively. The root mean squared error of the fit is 14 kHz. The angles between the internal rotor axis and principal axes are θ_a=12°, θ_b=78°, with θ_c held fixed at 90°.     

Microwave spectra, inversion splittings and quadrupole coupling constants of NHDCl and ND2Cl

The microwave spectra of monochloroamine (NH₂Cl) and its isotopic species have been observed by Cazzoli et al. [G. Cazzoli, D.G. Lister, P.G. Favero, J. Mol. Spectrosc. 42 (1972) 286-295; G. Cazzoli, D.G. Lister, J. Mol. Spectrosc. 45 (1973) 467-474]. We observed microwave spectra of four isotopic species of ¹⁴NHD³⁵Cl, ¹⁴NHD³⁷Cl, ¹⁴ND₂³⁵Cl, and ¹⁴ND₂³⁷Cl produced by the direct reaction of ammonia gas-d₃ or ammonium hydroxide-d₅ with N-chlorosuccinimide. The microwave spectra of NHDCl (d₁-species) and ND₂Cl (d₂-species) were observed in the frequency range from 8.0 to 60 GHz. The inversion splitting (ΔE_o) of ¹⁴NHD³⁵Cl and ¹⁴NHD³⁷Cl in the ground vibrational state are shown to be 11.46(15) and 11.44(15) MHz for K_a = 0 ← 1, and 10.49(15) and 10.26(15) MHz for K_a = 1 ← 2, respectively. However, the inversion splitting of the d₂-species could not be observed in our spectrometer. Only small J and K-dependence of the inversion splitting of d₁-species was observed. The rotational constants of ¹⁴NHD³⁵Cl were determined to be A = 187895.44(18), B = 13353.343(15) and C = 12859.794(15) MHz for the 0⁺ ← 0⁻ state, which means the transition from the lower inversion level to the upper one, and A = 187918.52(18), B = 13353.345(15) and C = 12859.798(14) MHz for the 0⁻ ← 0⁺ state. The rotational and centrifugal distortion constants of ¹⁴ND₂³⁵Cl were determined to be A = 141030.885(72), B = 12594.481(6) and C = 12055.356(6) MHz, and Δ_J = 18.342(23), Δ_JK = 318.15(56), Δ_K = 2219.3 (fixed), δ_J = 0.8717(17) and δ_K = 157.78(61) kHz. The values of the planar moments P_bb = (I_b − I_a − I_c)/2, of ¹⁴ND₂³⁵Cl and ¹⁴ND₂³⁷Cl were found to be 2.68898(2) and 2.68890(2) u Å², respectively, which are about twice as large as those of normal species (P_bb = 1.3548(6) and 1.3544(16) u Å², respectively). It was found that the bond length of r(N-Cl) of NH₂Cl was longer than that of Cl-NCO by 0.045(12) Å, and was almost the same as that of CH₂N-Cl, while it was much shorter than those of Cl-NO₂ and Cl-NO, by 0.092(6) and 0.227(6) Å, respectively.     

The microwave spectra and structures of epihalohydrins: Epichlorohydrin

The microwave spectrum of the two chlorine isotopic species of epichlorohydrin ($\text{CH}{}_2\text{OCH}$CH₂Cl) is reported. The structure is a gauche conformation with the Cl atom twisted toward the oxygen side of the ring. The observed rotational constants (in MHz) and centrifugal distortion constants (in kHz) are: C₂H₃OCH₂³⁵Cl; A = 13 373.02, B = 2080.353, C = 1932.469, Δ_JK = ? 6, Δ_K = 2400, δ_J = ? 0.43, δ_K = 17, H_KJ = ? 0.13, H_K = 570, h_JK = 0.061, h_K = ? 5.1: C₂H₃OCH₂³⁷Cl; A = 13 361.24, B = 2028.853, C = 1887.990, Δ_JK = 0.31, Δ_K = 1669., δ_J = ? 0.16, δ_K = 54.1.     

The microwave spectrum,structure, and quadrupole coupling constants of boron chloride difluoride,BCIF2

The microwave spectrum of boron chloride difluoride, BClF₂, has been investigated in the region 26.5–40.0 GHz. R-branch transitions belonging to the isotopic species ¹¹B³⁵Cl¹⁹F₂, ¹¹B³⁷Cl¹⁹F₂, and ¹⁰B³⁵Cl¹⁹F₂ have been observed and the derived rotational constants yield the following ground-state structural parameters: $\text{r}{}^0\text{(BF) = 1.315 ± 0.006}\text{A}\text{?}$, $\text{r}{}^{\mathrm{s}}\text{(BCl) = 1.728 ± 0.009}\text{A}\text{?}$, < FBF = 118.1 ± 0.5°. The ground-state rotational constants of the most abundant species ¹¹B³⁵Cl¹⁹F₂ are: A₀ = 10 449.32 ± 0.13, B₀ = 4705.811 ± 0.020, C₀ = 3239.702 ± 0.026 MHz, Δ_JK = 8.9 ± 1.7, and Δ_J = 1.86 ± 0.48 KHz. The asymmetry parameter κ = ?0.593291 and the inertial defect δ⁰ = 0.2361 amu Ų which is consistent with that expected for this type of molecule if planar. The ³⁵Cl quadrupole coupling constants for ¹¹B³⁵Cl¹⁹F₂ are χ_aa = ?42.8 ± 1.0, χ_bb = 30.2 ± 1.5, χ_cc = 12.6 ± 1.5 MHz with the asymmetry parameter η = 0.41.     

Microwave spectrum,conformation, quadrupole coupling constants,and barrier to internal rotation of methoxyamine

The microwave spectra of three isotopic species of methoxyamine (CH₃ONH₂) have been studied. For the normal species the ground-state rotational constants are A = 42488 ± 150 MHz, B = 10049.59 ± 0.03 MHz, and C = 8962.85 ± 0.03 MHz. From these data and those from the -NHD and -ND₂ species, the amino protons have been shown to occupy a symmetrical trans position relative to the methyl group. The barrier to internal rotation of the methyl group has been found to be 873 ± 15 cm^?1 by analysis of ground-state splittings. Analysis of hyperfine splittings has yielded the ¹⁴N quadrupole coupling constants, which have the following values for the normal isotopic species: χ_aa = 3.63 ± 0.03 MHz, χ_bb = ?3.69 ± 0.07 MHz, and χ_cc = 0.06 ± 0.07 MHz.     

The millimeter wave rotational spectrum of N-cyanomethanimine,CH2NCN

The rotational spectrum of the short-lived species N-cyanomethanimine, CH₂NCN, has been measured in the frequency range 100–250 GHz. The observed transitions allow the determination of the rotational and centrifugal distortion constants and the nitrogen quadrupole coupling constants for both nitrogen nuclei. The N-cyanomethanimine spectrum was measured directly in the products of the pyrolysis of trimethylenetetrazole. The rotational constants obtained are A = 63 372.995(11) MHz, B = 5 449.347 90(28) MHz, and C = 5 009.559 86(29) MHz; the quadrupole coupling constants are χ_aa = 2.057(39) MHz and χ_bb ? χ_cc = ?7.205(21) MHz for the imine nitrogen, and χ_aa = ?3.264(33) MHz and χ_bb ? χ_cc = ?1.630(18) MHz for the cyano-group nitrogen. The accurate constants obtained allow the calculation of the line position and hyperfine structure of any rotational transition appropriate for a radioastronomical search.     

The high resolution rotational spectrum of 2-cyanoaziridine

The microwave spectrum of 2-cyanoaziridine has been measured and assigned. Only the spectrum of the cis isomer has been detected although extensive searches for transitions from the trans isomer have been made, suggesting that it is at least 11 kJ mol^?1 less stable. The following nuclear quadrupole coupling constants were obtained: χ_aa(1) = ?1.249(14), χ_bb(1) = 1.407(12), χ_aa(2) = ?3.547(6), χ_bb(2) = 1.865(8) MHz, the former pair probably referring to the amino nitrogen and the latter pair to the nitrile nitrogen. The rotational constants derived from the analysis are: A = 16877.718(32), B = 3528.931(4), C = 3373.065(4) MHz, D_N = 1.10(4) kHz.     

Measurements of rotational transitions and hyperfine structure in PH2D

New rotational transition frequencies and measurements of hyperfine structure on two transitions are reported for PH₂D. All observed transitions are Q branch (ΔJ = 0) so only two independent rotational constants are obtained. These are A-C = 46 593.44 ± 0.67 MHz and κ(A-C) = 2B-A-C = ?34 545.9 ± 1.3 MHz. Nine transitions were fit to these parameters and the distortion parameter D_JK to obtain D_JK = 4.30 ± 0.04 MHz. Hyperfine structure due to spin-rotation interactions was observed on the 1₁₀ ← 1₁₁ transition at 6 024.645 MHz and on the 4₁₄ ← 4₀₄ transition at 20 815.334 MHz. Spin-rotation tensor components obtained are $\text{(M}{}_{\mathrm{aa}}\text{+ M}{}_{\mathrm{bb}}\text{)}\text{2}\text{=}\text{(M}{}_{\mathrm{aa}}\text{+ M}{}_{\mathrm{cc}}\text{)}\text{2}\text{= ?98 ± 3}\text{kHz}$.     

Microwave spectrum of ethyl iodide: Nuclear quadrupole interaction and centrifugal distortion analysis

The microwave spectrum of ethyl iodide has been reinvestigated between 4 and 80 GHz. A total of 181 ground-state transitions with J ≤ 26 and $\text{F ≤}\text{57}\text{2}$ have been analyzed using numerical diagonalization of the quadrupole Hamiltonian. The following rotational and quadrupole coupling constants have been determined (in MHz): A′ = 29 116.321; B′ = 2979.5639; C′ = 2796.4520; χ_aa = ?1478.111; χ_bb = 564.464; χ_cc = 913.648, and χ_ab = 896.38. The quadrupole coupling constants have been transformed to their principal axis system. All the quartic centrifugal distortion constants have been significantly determined, the standard deviation of the fit being only σ = 31 kHz.     

Nuclear quadrupole hyperfine quantum beats in HCl and pyrimidine measured by the pump-probe method using REMPI/LIF detection

Coherent time resolved IR-UV double resonance spectroscopy with REMPI and/or LIF detection has been used to measure the nuclear hyperfine structure of a diatomic and a polyatomic molecule. The pump-probe technique was applied and the experimental set up was optimized to achieve highest spectral resolution. Following excitation of the HCl fundamental vibrational transition by a nanosecond IR laser pulse, the nuclear quadrupole coupling constants were determined to be eQq = −69.51(22) MHz for H³⁵Cl and eQq = −54.40(16) MHz for H³⁷Cl in the J = 1 and J = 2 states of the υ = 1 level. Nuclear (Cl) spin-rotation interaction was shown to be active with the corresponding coupling constant being C _I = 0.068(10) MHz for H³⁵Cl and C _I = 0.049(8) MHz for H³⁷Cl. For pyrimidine a C-H stretch vibration ν ₁₃ was excited and the quadrupole tensor elements for the rovibronic states J _{K a} ,K _c = 1₁₀ and 1₀₁ of the υ ₁₃ = 1 level were found to be χ _aa = −3.095(10) MHz, χ _bb = 0.227(10) MHz and χ _cc = 3.322(10) MHz. In this case the residual frequency error was reduced to 8 kHz. The results of these jet experiments independently confirm those from millimeter wave and microwave measurements on static gas samples.     

Microwave spectrum of fluoroacetyl chloride

The microwave spectrum of fluoroacetyl chloride has been studied in the 8–40 GHz region and transitions arising from one conformer have been assigned. This conformer has all the heavy atoms in a plane with the fluorine and chlorine atoms trans to one another. The rotational constants and nuclear quadrupole coupling constants for the ground vibrational state are (in MHz): H₂FCCO³⁵Cl: A = 9025.82, B = 2403.92, C = 1920.70, χ_aa = ?47.7, χ_bb = 23,7, χ_cc = 24.1; H₂FCCO³⁷Cl: A = 8994.95, B = 2342.24, C = 1879.75, χ_aa = ?38.0, χ_bb = 18.9, χ_cc = 19.1. The spectrum of the first excited torsional state has been assigned. Some lines possibly due to a second conformer have been observed but not yet assigned.     

Quadrupole hyperfine structure of the rotational spectrum of aminoacetonitrile

From high-resolution studies of the microwave spectrum of aminoacetonitrile we have established the quadrupole coupling constants of both nitrogen atoms in the molecule. They are χ_aa = ?2.77 (0.04) MHz, χ_bb = 1.20 (0.09) MHz for the amino nitrogen, χ_aa = ?3.48 (0.03) MHz, χ_bb = 1.50 (0.06) MHz for the nitrile nitrogen. Improved values for rotational constants and centrifugal distortion constants also emerge from the present spectral analysis.     

Microwave spectrum,conformation, and quadrupole coupling constants of cyclopentyl chloride

The microwave spectra of the normal and two isotopic species of cyclopentyl chloride have been observed and analyzed. For the normal isotopic species the rotational constants (in MHz) are A = 4547.77 ± 0.01, B = 2290.22 ± 0.01, and C = 2073.34 ± 0.01. From the rotational constant data, it has been shown that the stable molecular conformation is the bent axial form. Quadrupole coupling constants have been measured for the ³⁵Cl nucleus, the values being (in MHz) χ_aa = ?23.70 ± 0.10, χ_bb = 32.33 ± 0.36, and χ_cc = ?8.63 ± 0.37. When transformed to the CCl bond axis system, the coupling constants confirm the axial structure. Extensive vibrational satellite structure, presumably arising from the pseudorotational ring mode with a fundamental frequency of 52 ± 5 cm^?1, has been observed and assigned. No spectral evidence has been observed for a second stable molecular conformer.     

Microwave spectrum,quadrupole coupling constants,and barrier to internal rotation of 2-iodopropene

The microwave spectrum of 2-iodopropene has been investigated between 7.7 and 18 GHz. The measured transition frequencies of the ground and two vibrationally excited states have been analyzed using direct diagonalization of the rotational and quadrupole Hamiltonian. The following rotational and quadrupole coupling constants have been determined in a leastsquares fit for the ground state: A = 9285.153(20) MHz; B = 2337.2198(14) MHz; C = 1887.5871(14) MHz; and χ_cc = ?1820.783(33) MHz; χ_ab = 147.5(10) MHz; χ_bb = 957.018(41) MHz; and χ_cc = 863.765(40) MHz. The quadrupole coupling constants have been transformed to their principal axis system. From the splittings of some transitions of the first torsionally excited state a value of V₃ = 905(3) cm^?1 has been found for the threefold barrier hindering the internal rotation of the methyl group.