Spectroscopic Techniques and Hindered Molecular Motion

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Krupnov was greatly strengthened.

Molecular Spectroscopy

This was made possible in part by the lifting of the iron curtain. Belov, M.

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Tretyakov, and E. One of the interesting results of these studies was that the internal rotation barrier of the CH 3 rotor was substantially lowered, a factor of two or more compared to free methanol.

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Andrey and Igor were hardware and software specialists, respectively, that had more than a thorough understanding of using computers to automate instrumentation. In the metal oxide experiments, a number of strong transitions were observed in the several GHz that were scanned. It turned out that these transitions belonged to the metal dioxides, which had never been seen previously in the gas phase.

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Suenram and Lovas entered a major long-term collaboration with the rotational spectroscopist, Jens-Uwe Grabow, from Kiel, Germany. One of the most noteworthy studies Jens carried out at NIST was on the very weakly bound rare gas dimer Ne-Ar which required a 1 Watt amplifier to boost the microwave input power to the cavity Gra Jens and Yoshi Kawashima, another frequent visitor to the NIST lab, also worked on the spectrum of conformers I and II of glycine to determine the dipole moments and resolve the 14 N hyperfine structure.

In these studies, glycine was produced both in a heated nozzle and by laser vaporization Lov95a.

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Jens was also principal author on the FTMW study of N 2 O 5 which exhibited large amplitude motions from the two NO 2 groups which undergo internal rotation tunneling via a geared rotation of the two units about their C 2 axes Gra Jens also developed an automated software package for control of the FTMW type instruments but one of the main collaborations at NIST was working with Suenram on the development of a scaled down smaller version of the FTMW instrument that was totally transportable.

One of these instruments was taken to Aberdeen, Maryland and installed in an Army surety laboratory and used to observe and analyze the rotational spectra of several of the nerve agents for the first time, first Sarin Wal01 , and then Soman Sue This instrument and one of the sister instruments at NIST were also used to study a large number of agent related compounds from to with support from the Army Research Office.

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DaBel04, Sue Another study by Suenram and coworkers was on dimethyl methylphosphonate which is among the organo-phosphorus compounds which are relatively harmless but serve as model compounds in place of nerve agents. The spectrum showed an unexpected complication in that the methoxy groups tunneled to produce equivalent structures in addition to the internal rotation of the three methyl groups Sue In the initial report only the A-state was fit, but later a global analysis was carried out Oha One could overlay a theoretical spectrum from assumed or ab initio calculated rotational constants on the laboratory scan.

The assumed rotational constants, dipole moment components, and resulting spectra are all computer mouse controlled by track bars. This literally reduced assignment times to sometimes a few minutes for spectra that might otherwise have taken days or weeks to analyze. Using this program, one can simulate up to nine separate spectra at once. Also, once a particular spectrum conformer has been assigned, it can be digitally subtracted from the overall spectrum to simplify the remaining spectra.

Spectroscopic Techniques and Hindered Molecular Motion | Taylor & Francis Group

This is true not only for multiple conformers, but isotopomers as well 13 C species for instance. Needless to say, it would have been difficult if not impossible to study and assign the spectra of quite a few of the larger molecules that we have studied without the use of this program, e. This was the last time the group used the 12 m, since in June NRAO turned over the operation of the instrument to the Astronomy Department of the University of Arizona. Hol01 showed that the spatial extent exceeded 60" rather than being confined to the hot core about 5" in diameter of the cloud Sgr B2 N-LMH.

From the same data taken in May , Hollis and co-workers identified ethylene glycol antifreeze , which is the sugar alcohol of glycolaldehyde Hol Shortly after the glycolaldehyde detection, Lovas spent six months at the Astronomy Department of the University of Illinois working with Lew Snyder and his students.

Meanwhile at NIST, Fraser, Suenram and a visiting scientist Cathrine Lugez embarked upon a series of studies involving 1-alkenes which were known to have large numbers of molecular conformers in room temperature gas phase samples. It was surmised that in a molecular beam, a much simpler picture would emerge as all the higher energy conformers would cool down to the lowest energy conformer and only the spectrum of the lowest energy conformer would be seen in the molecular beam.

The authors were surprised by the results in that the spectrum that was observed was quite complex, indicating that a number of conformers were present in the molecular beam.

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While it is true in some cases that conformers will freeze out, if there are barriers to internal rotation that inhibit a higher energy conformer from getting to a lower energy conformer, the individual conformers will remain suspended in the gas phase and emerge from the nozzle unscathed. While the alkanes were of primary interest, they tend to have very small dipole moments and thus are not amenable to microwave studies.

The first study reported on seven conformational isomers of the expected 13 for 1-hexene Fra This was followed with the observation of four of the five expected conformers of 1-pentene Fra00a. A year later these authors reported the observation of fifteen conformers of 1-octene Fra Thus the geometry and internal dynamics of this complex was of interest.

Rich Lavrich joined the group as a postdoctoral fellow from to He initially worked with Suenram, but when Suenram retired Lavrich was supervised by David Plusquellic.

Spectroscopic Techniques and Hindered Molecular Motion

Together they carried out a number of studies on conformational analysis of peptide mimetics. The first of these was on the alanine dipeptide analog N -acetyl-alanine N '-methylamide Lav With three methyl rotors, 14 torsional states are expected. However only three of these were analyzed, the AA, AE and EA states which accounted for the vast majority of the spectral features observed. A second study Rich carried out was on the peptide mimetic ethylacetamidoacetate also known as N -acetylglycine ethyl ester that yielded two low energy conformers Lav03a. Lovas and co-workers identified cyanoallene toward the TMC-1 molecular cloud aided by its hyperfine structure Lov The authors suggested a formation route for cyclopropenone via the addition of an oxygen atom to cyclopropenylidine c-C 3 H 2 as shown here:.

This is the largest symmetric top molecule detected to date. This is the largest interstellar molecule with a peptide linkage, i.

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Summary. Spectroscopic Techniques and Hindered Molecular Motion presents a united, theoretical approach to studying classical local thermal motion of small. Download Citation on ResearchGate | Spectroscopic techniques and hindered molecular motion | rystals by spectroscopic techniques. Mono- and.

The last two parts of the series "Microwave Spectra of Molecules of Astrophysical Interest" were published in and Returning to laboratory studies, several interesting polyols with three carbon atoms were studied as potential interstellar molecules. From the FTMW spectral scans 5 conformers were identified with the aid of ab initio calculations.

One of these showed an interesting tunneling between its two chiral forms Ily08a. The second study was on 1,2-propanediol. It is an asymmetric top molecule with at least 23 low energy conformers. The seven lowest energy conformers were assigned with the aid of ab initio calculations. We thought this might be successful since it only differs from the known interstellar molecule ethylene glycol by an additional methyl group. The final study of these C 3 polyols was on 1,3-propanediol.

As in the previous case both cavity and broadband FTMW spectrometers were used. Ab initio calculations were carried out on the 8 lowest energy forms to aid the spectral analysis. In this case only the two lowest energy conformers were assigned and both exhibited tunneling between their two chiral forms mirror images.

At the moment only Lovas remains in the microwave lab, so before long this story will end for the microwave lab. However, Dave Plusquellic and Kevin Douglass have recently developed a chirped-pulse broadband terahertz spectrometer in the region GHz— GHz and demonstrated its sensitivity for trace gas sensing Ger Thus, future rotational spectroscopy at NIST will likely move to their laboratory. In the references below and cited in the text, nearly all of the lab studies reported up to about are given.

For a more complete list the reader is encouraged to search the Web of Science or similar databases for publications from individual staff members and their visitors. Apponi, D. Halfen, L. Ziurys, J. Hollis, A. Remijan, and F. Lovas, Astrophys. Balle and W. Flygare, Rev. Yu Tretyakov, and R. Suenram, Astrophys. Blum, H.

Heddrich, F. Lovas, R. Suenram, and A. Maki, J. Brugh, R.