Pulsed Molecular Beam Microwave Fourier Research Articles

ChemInform Abstract: The Microwave Spectrum of N-Methylsulfinylamine.

Abstract The microwave spectrum of N -methylsulfinylamine has been recorded using a pulsed molecular beam microwave Fourier transform spectrometer. The barrier to methyl internal rotation was determined to be 314.06(48) cal/mole. The quadrupole coupling constants are X aa = 0.8425(13) MHz, X bb = −2.7586(21) MHz, X cc = 1.9161(21) MHz, and X ab = 1.705(37) MHz.

The microwave spectrum and molecular structure of ethylisocyanate

We have remeasured the spectrum of the isotopically most abundant species of ethylisocyanate (12CH3 12CH2 14N12C16O), and assigned for the first time the spectra of all 13C-, 15N-, and 18O-monosubstituted isotopomers in their respective natural abundances employing pulsed molecular beam microwave Fourier transform spectroscopy. We used these data to determine the heavy atom r s and r o structure where the most interesting structural parameters, the C-N-C and N-C-O angles, were found to be 138·5° and 176°, with the oxygen atom trans to the ethyl group. Additionally we obtained improved values of the 14N quadrupole coupling constants (in MHz) χ aa = 2·5488(6), χ bb = -1·1717(8), and χ cc = -1·3771(8) of the 12CH3 12CH2 14N12C16O species. From the variation of the coupling constants with isotopic substitution, we also determined the angular position of the coupling tensor in the inertial axis system.

The Microwave Spectrum of 2,6-Lutidine, Analysis of Internal Rotation and 14 N Hyperfine Structure

Abstract The rotational spectrum of 2,6-lutidine, (CH3)2C5H3N, has been recorded between 6 and 26.5 GHz using pulsed molecular beam microwave Fourier transform spectroscopy. The rotational constants are A = 3509.7139(84) MHz, B = 1906.8639(101) MHz, and C = 1254.6215(14) MHz, the barrier to internal rotation of the two methyl groups is V3 = 1.1752 kJ/mol, their moments of inertia were found to be Iα = 3.0808(9) uÅ2 . The nitrogen nuclear quadrupole constants are χaa = +1.600(5) MHz, χbb = -4.572(3) MHz and χcc = +2.972(5) MHz.

The Rotational Spectrum of Bromacetonitrile, Measured with Stark Modulated and Microwave Fourier Transform Spectrometers

The ground state microwave spectra of two isotopomers of bromoacetonitrile, BrCH 2CN, have been observed with a conventional Stark modulated spectrometer in the frequency range 21-43 GHz and with a cavity pulsed molecular beam microwave Fourier transform spectrometer in the frequency range 8-18 6Hz. Apparent anomalies in the literature values of the rotational and centrifugal distortion constants have been resolved. Perturbations in both the Br and the 14N hyperfine structures have been observed and accounted for. The complete quadrupole coupling tensors of both these nuclei have been determined and are discussed in terms of the molecular geometry.

Nuclear Quadrupole Coupling, Spin-Rotation, and Spin-Spin Interaction in the Rotational Spectrum of Ethinylisocyanide

The rotational J = 1-0 transitions of nine different isotopomers of ethinylisocyanide, H-CC-NC, were measured with high resolution using a pulsed molecular beam microwave Fourier transform (MB-MWFT) spectrometer. The analysis of the complex hyperfine patterns yielded nuclear quadrupole, spin-rotation, and spin-spin interaction constants.

Microwave Spectrum and Structure of the 1,2-Difluorobenzene-Argon van der Waals Complex

Using a pulsed molecular beam microwave Fourier transform spectrometer, we measured the rotational spectrum of the 1,2-difluorobenzene-argon van der Waals complex in the range from 7 to 18 GHz. The rotational and centrifugal distortion constants were found to be A = l373.49072(9) MHz, B = 923.31007(10) MHz, C = 831.07477(6) MHz, Δ J = 2.1104(8) kHz, Δ JK = 2.372(3) kHz, Δ K = −1.911(3) kHz. δ J = 0.4744(4) kHz, and δ K = 1.785(9) kHz. The argon atom is placed 3.543 Å above the ring plane.

Rotational spectrum, structure, and chlorine nuclear quadrupole coupling constants of the van der Waals complex Ar–Cl2

The pure rotational spectrum of the van der Waals complex Ar–Cl2 has been observed between 5 and 14 GHz using a Balle–Flygare type pulsed molecular beam microwave Fourier transform spectrometer. Ten a-type rotational transitions of Ar–35Cl2, as well as five a-type rotational transitions of the mixed isotopomer Ar–35Cl37Cl, have been assigned. The rotational constants and quartic centrifugal distortion constants have been determined. Unlike its isovalent linear isomer Ar–ClF, Ar–Cl2 has been found to be a T-shaped complex. This confirms the result from electronic spectroscopy and is in accord with the atom–atom additive model. The distance from the Ar nucleus to the center of mass of the Cl2 subunit has been calculated to be 3.7190 Å for Ar–35Cl2 and 3.7184 Å for Ar–35Cl37Cl, respectively. The van der Waals stretch and bend force constants, as well as their corresponding harmonic vibrational frequencies, have been derived and compared with the theoretically predicted values. The nuclear hyperfine splittings due to the two chlorine nuclei have been resolved and the coupling constants have been accurately determined. For 35Cl these are (in MHz): χaa=54.8180(16), χbb=−110.7131(19), χcc=55.8951(19) in Ar–35Cl2; and χaa=54.8407(27), χbb=−110.706(20), χcc=55.866(20) in Ar–35Cl37Cl. For 37Cl the values are χaa=43.2111(29), χbb=−87.267(12), χcc=44.056(12) in Ar–35Cl37Cl. A value has been obtained for the nuclear quadrupole coupling constant in free diatomic chlorine.

The microwave spectrum, structure, and harmonic force field of the van der Waals trimer Ar2–OCS

The microwave rotational spectra of the van der Waals trimers Ar2–OCS, Ar2–OC34S, and Ar2–18OCS have been observed between 7.0 and 18.0 GHz using pulsed molecular beam microwave Fourier transform (MWFT) spectroscopy. Sixty-five transitions, both a and c type, have been assigned for Ar2–OCS, as well as 37 transitions and 19 transitions for Ar2–OC34S and Ar2–18OCS, respectively. The rotational constants and quartic centrifugal distortion constants have been determined. The trimer has been found to be a distorted tetrahedrally shaped highly asymmetric top (κ=∼0.377) with the OCS subunit lying in the Cs symmetry plane. An effective ground state structure has been determined. The Ar–Ar distance and the two ‘‘Ar–center of mass of OCS’’ distances are slightly longer than those in the corresponding dimers Ar2 and Ar–OCS, respectively. A harmonic force field analysis has been performed on the basis of the centrifugal distortion constants, to predict the van der Waals force constants and the corresponding vibrational frequencies. The Ar–Ar stretch force constant of the trimer is 0.008 42 mdyn Å−1, which compares favorably with 0.0078 mdyn Å−1 reported for the free dimer Ar2. A comparison of the geometry and spectroscopic properties of Ar2–OCS with those of Ar2 and of the Ar2–HX (X=F, Cl, CN) trimers is given.

Pulsed Molecular Beam Microwave Fourier Transform Spectroscopy of the van der Waals Trimer Ar2-CO2

The microwave rotational spectrum of the van der Waals trimer Ar2-CO2 has been observed between 7 and 18 GHz using a Balle-Flygare type pulsed molecular beam microwave Fourier transform (MWFT) spectrometer. Twenty-one transitions, all b-type, Ka + Kc = even, have been assigned. The rotational constants and centrifugal distortion constants have been determined. The trimer has been found to be a C2v distorted tetrahedral shaped highly asymmetric top (κ = ∼0.360). The effective Ar-Ar distance and the two Ar-C distances have been evaluated and have been found to be similar to the corresponding values reported for the Ar2 dimer and for Ar-CO2, indicating that pairwise additivity is the dominating influence in the trimer bondings. A harmonic force field analysis has been performed on the basis of the centrifugal distortion constants. The estimated Ar-Ar stretch frequency of Ar2-CO2 is 23.5 cm−1, which compares favorably with 25.7 cm−1 reported for the Ar2 dimer. A comparison of the structures and the van der Waals force constants of Ar2-CO2 with other reported Ar2-small molecule trimers is given.

Nitrogen nuclear quadrupole coupling constants and heavy atom substitution geometry of tert-butyl nitrite

The microwave spectra of tert-butyl nitrite, including all of its heavy-atom singly substituted isotopomers, have been observed with a pulsed molecular beam microwave Fourier transform spectrometer. The analysis resulted in the following substitution structure parameters: r(O tN) = 118.18(8) pm, r(NO b) = 141.75(8) pm, r(O bC α ) = 143.75(6) pm, r(C α C i) = 152.34(3) pm, r(C α C o) = 152.90(4) pm, ∡(O tNO b) = 111.6(2)°, ∡(NO bC α) = 111.8(2)°, ∡(O bC αC i) = 103.0(1)° ∡(O bC αC o) = 110.1(1)°, and τ(NO bC α C o) = 61.6(1)°. The molecule has an ab-plane of symmetry. All measurements on the isotopically labeled species have been made using the isotopomers in their natural abundances. From the nuclear quadrupole hyperfine structures of the normal isotopic species and the [ 13C i]-labeled species, the elements of the nitrogen coupling tensor and its orientation in the bond axis system of the molecule have been determined to be χ zz = −5.53(17) MHz, χ xx = +2.15(17) MHz, χ yy = +3.377(1) MHz, and θ( a, x) = 24.3(13)°. Thus, the z-axis of the coupling tensor is inclined to the bisector of the O tNO b-angle by about 16.5° toward the O tN-bond.

The microwave spectrum, spin-rotation coupling constants, and structure of thiocarbonyl fluoride, SCF2, observed with a cavity microwave Fourier transform spectrometer

The microwave spectrum of thiocarbonyl fluoride, SCF2, has been investigated in the frequency range of 6.7–18.4 GHz using a pulsed molecular-beam microwave Fourier transform spectrometer. Twelve a-type transitions of the 32S12C19F2 isotopomer with J between 0 and 9, including ΔKa=2 transitions, as well as eight transitions of the 34S12C19F2 isotopomer with J between 0 and 9, have been assigned. The rotational constants and quartic centrifugal distortion constants have been determined. A 19F nuclear spin-rotation interaction, as well as a rather large direct 19F nuclear spin–spin interaction, have been observed. The three diagonal elements of the spin-rotation tensor at the 19F nuclei have been determined to be Maa=22.9(9) kHz, Mbb=10.4(4) kHz, and Mcc=5.4(3) kHz for 32S12C19F2 and Maa=20.9(9) kHz, Mbb=10.7(4) kHz, Mcc=6.4(4) kHz for 34S12C19F2, respectively. For 32S13C19F2, seven transitions with J between 0 and 6 have been assigned and rotational constants and two quartic centrifugal distortion constants have been derived. The splitting patterns due to the two 19F nuclei and the 13C nucleus have been observed but not completely resolved. A ground-state average structure has been determined for 32S12C19F2 as rz(CS)=1.5870(20) Å, rz(CF)=1.3182(13) Å, and ∢(FCF)=106.89(11)°.

The microwave spectrum of N-methylsulfinylamine

The microwave spectrum of N-methylsulfinylamine has been recorded using a pulsed molecular beam microwave Fourier transform spectrometer. The barrier to methyl internal rotation was determined to be 314.06(48) cal/mole. The quadrupole coupling constants are X aa = 0.8425(13) MHz, X bb = −2.7586(21) MHz, X cc = 1.9161(21) MHz, and X ab = 1.705(37) MHz.

The microwave spectrum of tert-butyl isocyanate

The rotational spectrum of tert-butyl isocyanate, (CH 3) 3C-NCO, has been assigned using pulsed molecular beam microwave Fourier transform spectroscopy. The rotational constants are A = 4572.6 MHz (assumed), B = 1632.948 (15) MHz, C = 1630.493 (14) MHz; the barrier to internal rotation of the tert-butyl group (assumed to constitute a symmetric internal rotor, the NCO group forming the frame) is V 3 = 41.510 (15) cm −1, the moment of inertia of the internal top is I α = 111.388 (4) amu · A ̊ 2 , and the angle between the internal rotor axis and the a-principal inertia axis is θ = 24.169 (3)°. The nitrogen nuclear quadrupole coupling constants are χ aa = +2.6376 (4) MHz, χ bb = −1.4935 (7) MHz, χ cc = −1.1441 (7) MHz, χ ac = +0.1731 (15) MHz (the molecule has an ac-plane of symmetry). The principal elements of the coupling tensor are χ zz = +2.6455 MHz, χ xx = −1.4935 MHz, χ yy = −1.1520 MHz; the angle between both axis systems is 〈( az) = +2.615° (with the x-axis perpendicular to the symmetry plane).

The Rotational Spectrum of the Fluorobenzene-Argon Van der Waals Complex

Abstract The rotational spectrum of the fluorobenzene-argon complex has been studied in the microwave region between 7 and 18 GHz using a pulsed molecular beam microwave Fourier transform spectrometer. The rotational constants were found to be A = 1811.81369(11) MHz, B= 1105.12965(15) MHz, C = 901.84281(5) MHz, the centrifugal distortion constants are ΔJ = 2.6886(17) kHz, ΔJK = 8.3761 (52) kHz ΔJK= -8.278(5) kHz, ρJ, =0.65993(72) kHz, and ρK = 6.013(11) kHz. The argon atom is placed 3.55 Å above the ring plane

14N Nuclear Quadrupole Hyperfine Structure in the Rotational Spectrum of Thionyl Aniline

Abstract Nuclear quadrupole hyperfine structures have been resolved in the rotational spectrum of thionyl aniline, C6H5NSO, using pulsed molecular beam microwave Fourier transform spectroscopy. High precision nuclear quadrupole coupling constants, rotational and quartic centrifugal distortion constants have been determined from the analysis of 12 low-J transitions. Coupling constants are χaa = + 1.5730(14) MHz and (χbb - χcc)= -5.6499(13) MHz. rotational constants are A-4026.72i5(4) MHz, B = 860.64732(8) MHz, and C = 709.52027(7) MHz, and centrifugal distortion constants are ΔJ - 36.6(5) Hz, ΔJK= -107.5(20) Hz, ΔK = 703(68) Hz, δJ = 8.1(5) Hz, and δK=111(19) Hz (representation I' used).

A Molecular Beam Microwave Fourier Transform (MB-MWFT) Spectrometer with an Electric Discharge Nozzle

Abstract We report on first experiments with our pulsed molecular beam microwave Fourier transform (MB-MWFT) spectrometer using a special nozzle which allows high voltage discharges within the nozzle orifice. Under these conditions we observed low J rotational transitions in highly excited vibrational states of carbonyl sulfide and sulfur dioxide, and also rotational lines of the SO radical in the 3Σ- electronic ground state and both the vibrational ground and first excited state.

The Microwave Spectrum of 1,1-Dichloro-2,2,2-Trifluoroethane

Abstract We assigned the microwave spectrum of 1,1-dichloro-2,2,2-trifluoroethane in its vibrational ground state using a pulsed molecular beam microwave Fourier transform (MB-MWFT) spectrometer. For the 35Cl, Cl-isotopomer the rotational constants were found to be A = 2105.7098(38) MHz, B = 1689.8304 (35) MHz, and C = 1135.6955 (23) MHz, the quadrupole coupling constants of the 35Cl-nuclei are χaa = 12.385 (35) MHz and χbb= -42.284(31) MHz. The rotational constants of the 35Cl,37Cl-isotopomer are A = 2071.5469 (33) MHz, B = 1670.7648 (36) MHz, and C = 1117.1755 (24) MHz, the corresponding quadrupole coupling constants are χaa = 17.639 (51) MHz and χbb= -47.670(67) MHz for the 35Cl-nucleus and χaa = 4.701 (54) MHz and χbb= -28.331 (83) MHz for the 37Cl-nucleus

The Microwave Spectrum of 4-Methylisothiazole

The microwave spectrum of 4-methylisothiazole was assigned by means of a pulsed molecular beam microwave Fourier transform spectrometer. Analysis of the spectrum yielded the rotational constants .4 = 7370.642(31) MHz, B = 2515.8461 (30) MHz, C = 1897.2863(33) MHz, a threefold barrier to internal rotation of the methyl group of V3 = 3170.81 (130) GHz≙ 105.7669(430) cm-1, a momentum of inertia of the methyl group of Iα = 3.16769(130) amu · Å2 and an angle between internal rotor axis and principal a-axis of Θ= 5.406(38)°. The nitrogen quadrupole coupling constants were determined to be χ+ = χbb + χcc = 1.4822(14) MHz, χ -= χbb-χcc = - 1.4353(25) MHz and χab= - 3.16(12) MHz.

The rotational spectrum and dipole moment of 1,1,1,2,3,3,3-heptafluoropropane in its vibrational ground state as observed by pulsed molecular beam microwave Fourier transform spectroscopy

The microwave spectrum of 1,1,1,2,3,3,3-heptafluoropropane has been recorded between 7 and 15 GHz using a pulsed molecular beam microwave Fourier transform spectrometer. The rotational constants were determined to be A = 2097.19052(7) MHz, B = 1054.56688(5) MHz, and C = 929.77833(4) MHz; the quartic centrifugal distortion constants are Δ J = 0.05319(15) KHz, Δ JK = 0.52269(40) kHz, Δ K = −0.45888(66) kHz, δ J = 0.00769(5) kHz, and δ K = −0.18586(136) kHz. In addition, the dipole moment components have been determined from the Stark effect to be μ b = 0.083(33) D and μ c = 1.4286(16) D.

The Microwave Spectrum and Dipole Moment of Hexafluoropropanone

Abstract We recorded the microwave spectrum of hexafluoropropanone between 7 and 15 GHz using a pulsed molecular beam microwave Fourier transform spectrometer. The rotational constants were determined to be A = 2181.71980(14) MHz, B= 1037.22930(7) MHz, C = 934.89233(8) MHz, the quartic centrifugal distortion constants are D'J= 0.07378 (39) kHz, D'JK = 0.10002(75) kHz, D'K = -0.07269(266) kHz, δ'J = 0.00623(29) kHz and R' 6= 0.00755(12) kHz. Stark effect measurements yielded a dipole moment μ = μb= 0.3949 (18) D