The microwave spectrum of synthesized molecule, 1-sila-1-isocyanocyclopent-3-ene has been studied using chirped pulse and cavity Fourier transform microwave (CP-FTMW and FTMW) techniques. The rotational spectrum has been assigned along with hyperfine splitting due to the ¹⁴N nucleus. Very limited sample could be synthesized at a time with this molecule, demonstrating the power of the CP-FTMW technique. Analysis of the molecule, along with structural possibilities will be discussed.

The rotational spectrum of diiodosilane was measured with a jet-pulsed, cavity Fourier transform microwave spectrometer over the frequency range 8.8 GHz to 15 GHz and assigned for the first time. The complete nuclear quadrupole coupling (NQC) tensors for both iodine nuclei were obtained for the ²⁸, ²⁹, and ³⁰ isotopologues of diiodosilane. In addition to the nuclear quadrupole coupling constants (NQCCs), rotational constants, centrifugal distortion constants, and nuclear-spin rotation constants were determined for each silicon isotopologue. Subtle, yet unmistakable, changes in the NQCCs of iodine upon isotopic substitution will be examined. A r₀ structure of diiodosilane was also fit via isotopic substitution, leading to the determination of bond lengths and angles: = 2.4236(19) Å, = 1.475(21) Å, ∠() = 111.27(13)^°, and ∠() = 105.9(19)^°. These results will be compared to the results of a previous gas electron diffraction study.

Large amplitude tunneling motions in (H₂S)₂ complicate the analysis of its microwave spectrum. The previous rotational spectrum of (H₂S)₂ was observed using the Balle-Flygare pulsed nozzle FT microwave spectrometers at NIST and IISc. For most isotopomers of (H₂S)₂ a two state pattern of a-type K_a=0 transitions had been observed and were interpreted to arise from E₁^+/− and E₂^+/− states of the six tunneling states expected for (H₂S)₂. K_a=0 lines gave us only the distance between the acceptor and donor S atoms.F. J. Lovas, P. K. Mandal and E. Arunan, unpublished work P. K. Mandal Ph.D. Dissertation, Indian Institute of Science, (2005) F. J. Lovas, R. D. Suenram, and L. H. Coudert. 43rd Int.Symp. on Molecular Spectroscopy. (1988) The (B+C)/2 for E₁ and E₂ states were found to be 1749.3091(8) MHz and 1748.1090(8) MHz respectively. In this work, we have observed the K_a=1 microwave transitions which enable us to determine finer structural details of the dimer. The observation of the K_a=1 lines indicate that (H₂S)₂ is not spherical in nature, their interactions do have some anisotropy. Preliminary assignment of K_a=1 lines for the E₁ state results in B=1752.859 MHz and C=1745.780 MHz. We also report a new progression of lines which probably belongs to the parent isotopomers.

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Footnotes:

F. J. Lovas, P. K. Mandal and E. Arunan, unpublished work

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Footnotes:

F. J. Lovas, R. D. Suenram, and L. H. Coudert. 43rd Int.Symp. on Molecular Spectroscopy. (1988).

The microwave spectra of the heteroaromatic molecules 4-bromopyrazole and 4-iodopyrazole have been recorded for the first time, along with their N-deuterated isotopologues. These species have recently been found to be useful in structural determination of proteins due to their ability to attach at a variety of binding sites.J. D. Bauman, J. J. E. K. Harrison, and E. Arnold, IUCrJ 2016, 3, 51-60he nuclear quadrupole coupling constants have been fitted, and these have been used to determine the nature of the C-X bond, and related to the strength of the halogen bonds formed by the molecules.

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Footnotes:

J. D. Bauman, J. J. E. K. Harrison, and E. Arnold, IUCrJ 2016, 3, 51-60T

r0pt Figure As part of a series of bromofluorocarbon species and analogues, the microwave spectrum of the molecule 3-bromo-1,1,1,2,2-pentafluoropropane has been measured on a CP-FTMW spectrometer located at Missouri S&T. The resultant spectrum is dense with transitions occurring at a rate of ≈ 1 transition/MHz! Within the spectrum, ⁷⁹Br and ⁸¹Br isotopologues of multiple conformers of 3-bromo-1,1,1,2,2-pentafluoropropane have been identified. Rotational constants, centrifugal distortion parameters, nuclear quadrupole coupling constants and how each compare with theory for each conformer will be discussed. Due to the large quadrupolar moment of bromine, heavy, brominated molecules are good candidates for dipole-forbidden transitions. Previous studies with bromoperfluoroacetoneF. E. Marshall, D. J. Gillcrist, T. D. Persinger, S. Jaeger, C. C. Hurley, N. E. Shreve, N. Moon, and G. S. Grubbs II, J. Mol. Spectrosc. 328 (2016) 59.rovided a rich spectrum full of dipole forbidden transitions that 3-bromo-1,1,1,2,2-pentafluoropropane does not share. This difference will be explained using structural considerations along with the matrix elements needed to enact these transitions.

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Footnotes:

F. E. Marshall, D. J. Gillcrist, T. D. Persinger, S. Jaeger, C. C. Hurley, N. E. Shreve, N. Moon, and G. S. Grubbs II, J. Mol. Spectrosc. 328 (2016) 59.p

The rotational spectrum of 2-bromopyridine () was reinvestigated in the frequency range of 10-15.5 GHz by high-resolution Fourier transform microwave (FTMW) spectroscopy. The new observations of ¹⁴ hyperfine splittings in previously studied transitionsCaminati, W.; Forti, P. Chemical Physics Letters 1972, 15(3), 343–349.elonging to both bromine isotopologues (⁷⁹ and ⁸¹) led to improved measurements of the rotational constants and bromine nuclear quadrupole coupling constants. The full nuclear quadrupole coupling (NQC) tensor of ¹⁴ was resolved for the first time, in addition to five centrifugal distortion constants. A comparison of the two ¹⁴ NQC tensors of ⁷⁹ and ⁸¹ will be presented.

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Caminati, W.; Forti, P. Chemical Physics Letters 1972, 15(3), 343–349.b

The ground state rotational spectra of the three methylamine substituted pyridines, 2-, 3-, and 4-picolylamine, were collected and analyzed over the frequency range of 7-17.5 GHz using chirped-pulsed Fourier transform microwave spectroscopy. All three molecules show a distinctive quadrupole splitting, which is representative of the local electronic environment around the two different ¹⁴N nuclei, with the pyridine nitrogen being particularly sensitive to the pi-electron distribution within the ring. The role that the position of the methylamine group plays on the quadrupole coupling constants on both nitrogens will be discussed and compared to other substituted pyridines.

The rotational spectra of three deuterated isotopologues of the dimer of formic acid have been measured, thank to the small dipole moment induced by asymmetric H - D substitution(s). For the HCOOH-DCOOH species the concerted double proton transfer of the two hydroxyl hydrogens takes place between two equivalent minima and generates a tunneling splitting of 331.2(6) MHz. From this splitting a barrier to proton tunneling of about 30 kJ/mol has been estimated.

In this talk we examine the two-dimensional tunneling formalism used previously to fit the hydrogen-transfer and internal-rotation splittings in the microwave spectrum of 2-methylmalonaldehyde in an effort to determine the origin of various counterintuitive results concerning the isotopic dependence of the internal-rotation splittings in that molecule. We find that the cause of the problem lies in a “parameter contamination” phenomenon, where some of the numerical magnitude of splitting parameters from modes with large tunneling splittings “leaks into” the parameters of modes with smaller tunneling splittings. We further find that such parameter contamination, which greatly complicates the determination of barrier heights from the least-squares-fitted splitting parameters, will be a general problem in spectral fits using the multi-dimensional tunneling formalism, since it arises from subtle mathematical features of the non-orthogonal framework functions used to set up the tunneling Hamiltonian. Transforming to a physically less intuitive orthonormal set of basis functions allows us to give an approximate numerical estimate of the contamination of tunneling parameters for 2-methylmalonaldehyde by combining a dominant tunneling path hypothesis with results recently given for the hydrogen-transfer-internal-rotation potential function for this molecule.

In a recent series of experimentsP. Jansen, L. Semeria, L. Esteban Hofer, S. Scheidegger, J. A. Agner, H. Schmutz, and F. Merkt, Phys. Rev. Lett. 114, 133202 (2015).,L. Semeria, P. Jansen, and F. Merkt, J. Chem. Phys. 145, 204301 (2016). we have determined term values of all rotational levels of the X^+ 2Σ_u⁺ (ν⁺=0) ground vibronic state of ⁴He₂⁺ with rotational quantum number N⁺ ≤ 19 at an accuracy of 25 MHz using MQDT-assisted Rydberg-series extrapolation of metastable helium molecules in the a ³Σ_u⁺ state. The precison of these experiments was limited by the 150 MHz linewidth of the pulsed laser system employed. In order to improve our resolution and possibly observe the spin-rotation splitting in the He₂⁺ ion, we have replaced the pulsed laser by a CW laser system with a bandwidth of 1.5 MHz. This system was used to measure the spin-spin and spin-rotation fine structure of metastable He₂ in the a ³Σ_u⁺ (ν"=0) state. Metastable helium molecules were produced by striking a discharge in an expansion of neat helium gas. By cooling the source to a temperature of 10 K, the velocity of the molecular beam was reduced to 500 m/s and an experimental Doppler-limited linewidth of 25 MHz was observed. Fine-structure splittings for all rotational levels with N" ≤ 27 have been measured at an accuracy of 5 MHz and, when possible, have been compared to the values reported in earlier investigations. W. Lichten, M. V. McCusker, and T. L. Vierima, J. Chem. Phys. 61, 2200 (1974).,W. Lichten and T. Wik, J. Chem. Phys. 69, 98 (1978).,M. Kristensen and N. Bjerre, J. Chem. Phys. 93, 983 (1990).,I Hazell, A. Norregaard, and N. Bjerre, J. Mol. Spectrosc. 172, 135 (1995).his comparison revealed a discrepancy that increased with increasing values of N". To verify our results, we have recently constructed a variaton of a classical molecular-beam magnetic-resonance setup that uses a multistage Zeeman decelerator and a RF stripline for de- and repopulation of the F₂ spin-rotational components with J"=N", respectively.

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Footnotes:

P. Jansen, L. Semeria, L. Esteban Hofer, S. Scheidegger, J. A. Agner, H. Schmutz, and F. Merkt, Phys. Rev. Lett. 114, 133202 (2015). L. Semeria, P. Jansen, and F. Merkt, J. Chem. Phys. 145, 204301 (2016)., W. Lichten, M. V. McCusker, and T. L. Vierima, J. Chem. Phys. 61, 2200 (1974). W. Lichten and T. Wik, J. Chem. Phys. 69, 98 (1978). M. Kristensen and N. Bjerre, J. Chem. Phys. 93, 983 (1990). I Hazell, A. Norregaard, and N. Bjerre, J. Mol. Spectrosc. 172, 135 (1995).T

The microwave spectra of two isomers of iodoimidazole have been recorded and assigned with resolution of their nuclear quadrupole coupling constants. These constants have been analysed in terms of the conjugation between the lone pairs on the iodine atom and the aromatic π-bonding system, and the effect of this conjugation on the distribution of π-electron density in the ring. A comparison of these properties has been made between iodoimidazole and other 5- and 6-membered aromatic rings bonded to halogen atoms.