Laboratory and tentative interstellar detection of trans-methyl formate using the publicly available Green Bank Telescope PRIMOS survey

The rotational spectrum of the higher-energy trans conformational isomer of methyl formate has been assigned for the first time using several pulsed-jet Fourier transform microwave spectrometers in the 6-60 GHz frequency range. This species has also been sought toward the Sagittarius B2(N) molecular cloud using the publicly available PRIMOS survey from the Green Bank Telescope. We detect seven absorption features in the survey that coincide with laboratory transitions of trans-methyl formate, from which we derive a column density of 3.1 (+2.6, -1.2) \times 10^13 cm-2 and a rotational temperature of 7.6 \pm 1.5 K. This excitation temperature is significantly lower than that of the more stable cis conformer in the same source but is consistent with that of other complex molecular species recently detected in Sgr B2(N). The difference in the rotational temperatures of the two conformers suggests that they have different spatial distributions in this source. As the abundance of trans-methyl formate is far higher than would be expected if the cis and trans conformers are in thermodynamic equilibrium, processes that could preferentially form trans-methyl formate in this region are discussed. We also discuss measurements that could be performed to make this detection more certain. This manuscript demonstrates how publicly available broadband radio astronomical surveys of chemically rich molecular clouds can be used in conjunction with laboratory rotational spectroscopy to search for new molecules in the interstellar medium.Comment: 40 pages, 7 figures, 4 tables; accepted for publication in Ap

CONFORMATIONAL ISOMERIZATION KINETICS OF VINYL ISOCYANATE MEASURED BY DYNAMIC ROTATIONAL SPECTROSCOPY

Author Institution: Department of Chemistry, University of Virginia, McCormick Rd., Charlottesville, VA 22904Vinyl isocyanate (CH$_2$=CH--N=C=O) can exist in two stable conformations, having a \textit{cis} or \textit{trans} arrangement about the C-N single bond. Electronic structure calculations indicate that the \textit{trans} conformer is about 250 cm$^{-1}$ lower in energy. There is a low barrier to isomerization of the \textit{trans} conformer calculated to be about 580 cm$^{-1}$. These energetics are supported by the pure rotational spectrum of vinyl isocyanate in a molecular beam, where transitions of the \textit{trans} conformer are about 300 times stronger than those of the \textit{cis} conformer due to conformational relaxation in the free-jet expansion. The dominance of the \textit{trans} conformer guarantees that vibrational excited states prepared by pulsed infrared laser excitation are effectively conformer-selective. Dynamic rotational spectra of laser-prepared excited states in the 3000 cm$^{-1}$ region of the spectrum show evidence of conformational isomerization through coalescence of the overall line shape. Strong mode-specific reaction yields are observed from different vibrational bands where, in some frequency regions, there is no evidence of conformational isomerization despite the molecule having a total vibrational energy greatly exceeding the barrier to reaction. The nuclear quadrupole hyperfine structure of the dynamic rotational spectra shows that isomerization, when it occurs, conserves the $K_a$ angular momentum projection quantum number

TUNNELING-SYMMETRY-RESOLVED VIBRATIONAL SPECTROSCOPY AND DYNAMICS OF THE C2_2H4_4-H2_2S COMPLEX MEASURED USING COHERENCE-DETECTED FOURIER TRANSFORM MICROWAVE (FTMW)--INFRARED SPECTROSCOPY

M. Goswami, P.K. Mandal, D.J. Ramdass, and E. Arunan, Chem. Phys. Lett.Author Institution: Department of Chemistry, University of Virginia, McCormick Rd., Charlottesville, VA 22904; Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, IndiaThe pure rotational spectrum of the C$_2$H$_4$-H$_2$S weakly bound complex displays a four-line pattern from the tunneling motions of the hydrogens in both the H$_2$S and C$_2$H$_4$ subunits.} 393 (2004) 22.} The rotational spectra of isotopic species show that the larger tunneling splitting (1.67 MHz) is attributed to motion of the H$_2$S subunit and the smaller (0.14 MHz) to tunneling of the ethylene unit. We extend the study of this complex through measurements of its hydride stretching fundamentals using coherence-detected FTMW--infrared spectroscopy. This technique permits background-free detection of the infrared spectrum through a three-pulse sequence (MW-IR-MW) that converts the laser-induced population difference to a detectable microwave coherence. By measuring the infrared spectrum through its resonance with the pure rotational transitions, we obtain vibrational spectra with rotational level and tunneling symmetry assignments. The observed changes in the tunneling splitting upon vibrational excitation are consistent with the assignments from the isotopic pure rotational measurements. At the experimental IR resolution (0.02 cm$^{-1}$) we are able to detect vibrational predissociation in the ``bound'' S-H stretching fundamental through Lorentzian broadening of the lineshape. The other vibrational bands (the ``free'' S-H stretch and the two infrared-active ethylenic C-H stretch fundamentals) have instrument-limited lineshapes indicating slower vibrational predissociation rates. The different selection rules for the two ethylenic C-H stretch fundamentals make it possible to estimate the energy difference between tunneling levels associated with the ethylene unit. We also observe some local perturbations in the vibrational spectrum and a vibrational band that is tentatively assigned as a combination band of the S-H stretch and a low-frequency van der Waals mode of the complex

SENSITIVITY LIMITS OF DEEP AVERAGE BROADBAND MICROWAVE AND MM-WAVE SPECTRA

Author Institution: Department of Chemistry, University of Virginia, McCormick Rd.; Charlottesville, VA 22904-4319High-speed digitizers have enabled the field of broadband molecular rotational spectroscopy at microwave-to-THz frequencies. Improvements in data throughput from these digitizers makes it feasible to perform deep averages (often more than 1 million time-domain averages of the free induction decay) to increase the measurement sensitivity. The use of broadband signal detection introduces new issues that are key for determining the practical sensitivity limits of these spectrometers. The practical limit on spectrometer sensitivity is often set by the number of spurious signals that are generated by the molecular signals themselves. For example, in cases where the molecular signals are down converted prior to digitization, the spectral purity of the local oscillator is crucial with spurious frequencies introducing spectral images. It is also possible to generate new local oscillator frequencies within the broadband mixers typically used in the broadband down conversion. A second issue it the potential for a vast number of intermodulation (IM) spurious signals resulting from the beating of two strong molecular transitions. This beat frequency can subsequently modulate all other molecular signals adding sidebands to all transitions at the beat frequency of the transition pair. This talk will summarize our experience with the spurious signal levels coming from these effects and the strategies we have adopted to minimize spurious signals in spectra where high sensitivity is necessary

COHERENCE-DETECTED FTMW-IR SPECTROSCOPY OF CH3_3OD IN THE OD STRETCH REGION.

Author Institution: Department of Chemistry, The University of Akron, Akron OH 44325; Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319, Charlottesville, VA 22904Infrared spectra of the connected rotational levels of jet-cooled CH$_3$OD are recorded in the OD stretch region. The observed spectra in the range 2710 - 2740 cm$^{-1}$ result from E-species transitions ($3_0\leftarrow 3_{-1}$,$2_0\leftarrow 3_1$,$1_0\leftarrow 1_1$) of CH$_3$OD. For the available rotational levels (K ${'}$ = 0 , K ${'}$ = 1 and K ${'}$ = 2), the reduced torsional energies follow a pattern similar to the ground state. The torsional tunneling splitting in the OD stretch excited state is deduced to be 2.4 cm$^{-1}$ as compared to 2.6 cm$^{-1}$ in the ground state

COHERENCE-CONVERTED POPULATION TRANSFER FTMW-IR DOUBLE RESONANCE SPECTROSCOPY OF CH3OD IN THE C-H STRETCH REGION

Author Institution: Department of Chemistry, The University of Akron, Akron OH 44325; Department of Chemistry, University of Virginia, McCormick Rd., Charlottesville, VA 22904Coherence-converted population transfer microwave-infrared double resonance spectroscopy is employed to record the rotationally state-selected infrared spectra of jet-cooled CH$_3$OD in the C-H stretch region (2750$-$3020 cm$^{-1}$). The observed infrared spectra result from the E-species microwave transitions (1$_0$ \leftarrow 1$_{-1}$ at 18.957 GHz, 2$_0$ \leftarrow 2$_{-1}$ at 18.991 GHz, and 3$_0$ \leftarrow 3$_{-1}$ at 19.005 GHz). The present spectra of CH$_3$OD contain 17 interacting vibrational bands ($J^{\prime}$ = 0). In additional to the three C-H stretch fundamentals ($\nu_3$:2841.7 cm$^{-1}$, $\nu_9:2954.4 cm$^{-1}$and$\nu_2:2998.9 cm$^{-1}$), 14 additional band origins are found in the region of the binary combinations of the CH bends (2890$-$2950 cm$^{-1}$). Although the A-species was inaccessible in the present work, the pattern of E-species reduced energies suggests that the torsional tunneling splittings of $\nu_3$ and $\nu_9$ are normal, whereas $\nu_2$ is inverted. The number and distribution of the observed vibrational bands support a stepwise coupling scheme in which the CH stretch bright state couples first to the binary C-H bend combinations, and then to all of the higher order vibrational combinations. A time-dependent interpretation in the asymmetric region indicates a fast (170 fs) initial decay of the bright state

MICROSOLVATION OF β\beta-PROPIOLACTONE AS REVEALED BY CHIRPED-PULSE FOURIER TRANSFORM MICROWAVE SPECTROSCOPY

Author Institution: Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319, Charlottesville, VA 22904; Grupo de Espectroscopia Molecular (GEM), Departamento de Quimica Fisica y Quimica Inorganica, Facultad de Ciencias, Universidad de Valladolid, E-47005 Valladolid, SpainMicrowave spectra of water clusters of $\beta$-propiolactone with up to five water molecules attached are presented. Helium or neon carrier gas with 3 atm of backing pressure is flowed over a room-temperature water reservoir, then over a room-temperature sample of $\beta$-propiolactone before being expanded into a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer operating between 6.5 and 18.5 GHz. A very dense spectrum, with approximately 2000 lines with a signal to noise ratio of at least 3:1, was observed, of which 800 have been assigned to a total of 20 species, including isotopomers in natural abundance and clusters with the carrier gas. Due to the complexity of the spectrum, after the first few, all other spectra were assigned with the aid of microwave-microwave double resonance experiments, either performed on the CP-FTMW spectrometer or in a Balle-Flygare-type cavity FTMW spectrometer. In the case of an extremely dense spectrum like this in which many species are present, these double resonance measurements are required to successfully analyze the spectrum. Stark effect measurements and assignments of isotopically substituted species are used to determine the structures of these microsolvated complexes