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Rotational Spectrum of Sarin
Journal of Molecular Spectroscopy 207, 77–82 (2001) doi:10.1006/jmsp.2001.8307, available online at http://www.idealibrary.com on
Rotational Spectrum of Sarin1 A. R. Hight Walker,∗ R. D. Suenram,∗ Alan Samuels,† James Jensen,† Michael W. Ellzy,† J. Michael Lochner,† and Daniel Zeroka‡ ∗ Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899; †Edgewood Research, Development, and Engineering Center, Aberdeen Proving Grounds, Aberdeen, Maryland 21010-5423; and ‡Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015-3172 Received December 19, 2000; published online April 3, 2001
As part of an effort to examine the possibility of using molecular-beam Fourier-transform microwave spectroscopy to unambiguously detect and monitor chemical warfare agents, we report the first observation and assignment of the rotational spectrum of the nerve agent Sarin (GB) (Methylphosphonofluoridic acid 1-methyl-ethyl ester, CAS #107-44-8) at frequencies between 10 and 22 GHz. Only one of the two low-energy conformers of this organophosphorus compound (C4 H10 FO2 P) was observed in the rotationally cold (Trot < 2 K) molecular beam. The experimental asymmetric-rotor ground-state rotational constants of this conformer are A = 2874.0710(9) MHz, B = 1168.5776(4) MHz, C = 1056.3363(4) MHz (Type A standard uncertainties are given, i.e., 1 σ ), as obtained from a least-squares analysis of 74 a-, b-, and c-type rotational transitions. Several of the transitions are split into doublets due to the internal rotation of the methyl group attached to the phosphorus. The three-foldsymmetry barrier to internal rotation estimated from these splittings is 677.0(4) cm−1 . Ab initio electronic structure calculations using Hartree–Fock, density functional, and Moller–Plesset perturbation theories have also been made. The structure of the lowest-energy conformer determined from a structural optimization at the MP2/6-311G∗∗ level of theory is consistent with our C 2001 Academic Press experimental findings. °
a database of microwave absorption frequencies and detection limits and a set of validated procedures for qualitative and quantitative chemical analysis. A reduction in the size, complexity, and cost of the instrument would also be valuable. Sarin is a challenge to modern spectroscopic techniques, since it is a molecule with eight heavy atoms, several predicted lowenergy conformers, and a large-amplitude motion. Virtually all molecules that display biological activity exhibit several conformers and have large-amplitude motions. The present paper is thus also part of a larger NIST effort aimed ultimately at the microwave investigation of secondary (folding) structures of biomimetics in the gas phase. Here, as a small step along that path, the rotational spectrum of Sarin is observed and assigned to a near rigid asymmetric rotor associated with the lowest energy conformational isomer of the molecule. This conformer has C1 point-group symmetry. Small splittings of the rotational lines, attributed to methyl-top internal-rotation tunneling about the C– P bond, imply a barrier to internal rotation of 677.0(4) cm−1 .
INTRODUCTION
The microwave spectrum of Sarin (GB) (Methylphosphonofluoridic acid 1-methyl-ethyl ester, CAS # 107-44-8) is of interest as part of an effort to develop microwave-based sensors for the detection of chemical-warfare agents. It is desired that such a sensor provide rapid, sensitive, portable, and speciesspecific quantitative detection of chemical warfare agents for use in treaty verification, safety monitoring at disposal sites, counter terrorism, and troop protection. Current detection technologies include ion-mobility spectrometry, flame photometry, gas chromatography, mass spectrometry, infrared spectroscopy, wet-chemical methods, surface-acoustic-wave devices, colorchange chemistry, and enzyme-based sensors (1). Each of these methods suffers from deficiencies in such areas as cost, specificity, complexity, false-alarm rate, reliability, and size. Here, we demonstrate the capabilities of Fourier-transform microwave (FTMW) spectroscopy (2) to detect chemical-warfare agents by observing the rotational spectrum of the relatively high vapor pressure (0.4 kPa at 298 K) nerve agent, Sarin. The present paper is part of a broader effort to develop FTMW spectroscopy for the detection, quantification, and realtime monitoring of various trace chemical species in air (3–5). For FTMW spectroscopy to be competitive against other techniques for the detection of trace gases it is necessary to develop
EXPERIMENTAL
To safely perform the experiments, a mobile FTMW spectrometer was installed in a surety laboratory at the U.S. Army Edgewood Chemical and Biological Center (ECBC). The capabilities of the staff at ECBC to synthesize Sarin and ensure its safe use were critical for the success of these experiments. A detailed description of the mobile FTMW spectrometer used for this project has been reported previously (5). Briefly, a
1 Throughout the course of this manuscript, certain trade names are necessary to adequately describe the software of the equipment used. The use of these trade names should not be construed as an endorsement by NIST in any way.
77
0022-2852/01 $35.00 C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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pulsed-molecular-beam of inert carrier gas seeded with Sarin vapor is injected coaxially into a high-Q (50,000–100,000) Fabry– Perot microwave cavity. The rotational temperature of the expansion of typically <2 K allows only one conformer of Sarin to be appreciably populated in the molecular beam. The Fabry–Perot resonator is housed in a vacuum chamber pumped by a diffusion pump to a base pressure of approximately 13 µPa (10−7 Torr). Once the molecular beam travels to the center of the cavity, microwave radiation, which is tunable from 10 to 26.5 GHz, is pulsed into the cavity via an L-shaped antenna mounted in the center of one of the mirrors. If a rotational transition lies within the 500-kHz bandwidth of the microwave cavity, the molecules are coherently excited. The free-induction decay signal produced by this coherent excitation is collected at a matched antenna in the second mirror and monitored by a superheterodyne receiver. The Fourier transform of the free-induction decay gives a spectrum from which the frequency of the rotational transition can be determined. Frequency measurements are referenced to the 10 MHz Loran-C frequency, which itself is referenced to a Cs clock with a stated standard uncertainty of 1 part in 1012 . For the work being described here, a flow nozzle (6) mounted on the integral end flange-mirror of the Fabry–Perot cavity was used to inject the gas into the chamber. A stainless steel inlet line transported a Ne/He (80%/20%) stream through a diffusion tube to collect small amounts of Sarin vapor prior to exiting from the nozzle. The 200-µl Sarin sample was kept in an exhaust hood during the experiments and only the dilute gas stream was flowed to the pulsed-molecular-beam valve. Flow controllers were used to maintain a flow rate of 20 to 30 cc/min through the nozzle, with the excess flow being routed back into the hood through a second stainless steel line. The exhaust gas from the roughing pump was also vented into the hood. To guide the microwave experiment, Molecular Mechanics (MM+) structural predictions were performed using HyperChem Version 4.5. Several conformers were predicted to lie at low energies, although the B + C rotational constants for the different conformers were similar (within 20%). All of the structures were predicted to have a significant projection of the electric dipole moment vector (µ > 3.3 × 10−30 C m [>1 Debye]) along the three principal inertial axes. The spectrum of each conformer of Sarin was expected to be that of a near prolate rotor, with a-type, R-branch transitions spaced by approximately B + C. The mobile FTMW spectrometer is not equipped with Stark plates for static electric-field measurements, so the rotational assignments were made by pattern recognition and verified through fitting to an asymmetric rotor internal-rotation Hamiltonian. ROTATIONAL ASSIGNMENT
Automated broadband survey spectra for Sarin were recorded at a scan rate of approximately 1 GHz/h. The spectral regions searched were guided by the molecular mechanics predictions for the positions of the a-type R-branch (i.e., 1J = 1, 1Ka = 0)
FIG. 1. Broad (2-GHz) survey scan of Sarin vapor. Entire scan took less than 3 h.
clumps. A sample of one such survey spectrum is shown in Fig. 1 for frequencies between 12 GHz and 14 GHz. The lines in the figure have been assigned to the lowest energy rotational or conformational isomer of Sarin. Searches in regions where transitions from other low-energy conformers are predicted were unsuccessful, demonstrating that only one conformer is populated in the rotationally cold molecular beam. We note that previous studies of conformational relaxation in cold molecular beams suggest efficient cooling when the barrier between the higher and lower energy conformer is less than 400 cm−1 (7). For higher barriers, the relative populations of the conformers are expected to be unchanged from the room-temperature distribution. The transitions measured in the low-resolution survey spectrum were recorded at high resolution, and the line frequencies were fit to a standard asymmetric-rotor Hamiltonian. Transitions were observed arising from all three asymmetricrotor electric-dipole selection rules (a-, b-, and c-type). A number of the lines were partially or, in some cases, fully split into equal-intensity doublets, as pictured in Fig. 2. This splitting is attributed to internal-rotation tunneling arising from one of the methyl tops of the Sarin molecule. The methyl tops on the isopropyl group are sterically hindered and are thus not likely to give rise to splittings of this magnitude. In fact, previous studies have shown that the high barriers to internal rotation for methyl tops in isopropyl groups do not give rise to observable splittings at the FTMW resolution (8). The observed doublet transitions are thus assigned to the tunneling-split A and E symmetry components arising from the internal-rotation tunneling of the methyl top attached to the phosphorus atom. Once the assignments were complete, a final data fit was made to an internal-rotation centrifugal-distortion Hamiltonian using a program written by Woods (9). The barrier to internal rotation of the methyl top attached to the phosphorus atom was determined to be V3 = 677.0(4) cm−1 . In addition, Coriolis interactions between the rotational and internal rotation angular momenta
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ROTATIONAL SPECTRUM OF SARIN
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TABLE 1 Observed Rotational Transitions for Sarin1
FIG. 2. Two rotational transitions that have been split into two components of A and E symmetry due to the internal rotation of the methyl group attached to the phosphorus.
allow the angle between the methyl-top axis and the a-principal inertial axis of the molecule to be determined as 62.70(4)◦ . The assignments and frequencies of the 52 transitions used in the final analysis are listed in Table 1. The resulting rotational constants and internal-rotation parameters are given in Table 2. THEORETICAL CALCULATIONS
Ten different Sarin trial geometries were optimized using the standard HF, DFT/B3LYP, and MP2 levels of theory with a 6-311G∗∗ basis set, as implemented in the Gaussian 94 electronic-structure program package (10). These 10 geometries, which differ by rotation about the P–O and Cisopropyl –O bonds, optimize to five unique conformers. We note that isoenergetic enantiomers of Sarin are possible via the interchange of the oxygen and fluorine positions on the chiral phosphorus. Table 3 lists the relative energies and rotational constants for the five isomers as determined by the three different calculations. Although the energy difference between the two lowest-energy conformers is small, approximately 0.2 kJ/mol, the conformer energy ordering is invariant with level of theory, with the HF and MP2 calculations closely agreeing. Note that the energy ordering is the same whether one takes the electronic energy, E el , or the vibrational-zero-point-corrected electronic energy, E el+zpe . Assuming thermal equilibrium and a 1 K molecular-beam conformational temperature gives a fractional population of less than 10−15 for the second lowest energy conformer. Previous studies (7) of conformational isomerization demonstrate minimal conformational cooling in supersonic expansions, except when the barrier to conformational relaxation is less than 400 cm−1 . From the experiments undertaken for this paper, which included spectral searches for a second conformer, we
1
The standard uncertainties on the transition frequencies are 2 kHz. These transitions are not included in the least-squares fit since they exhibit unresolved A/E torsional splittings. 2
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TABLE 2 Rotational Constants of Sarin (GB)
TABLE 4 Structural Comparison of the Two Lowest Energy Conformers: Bond Lengths and Angles Calculated at the MP2/6-311G∗∗ Level of Theory
a Type A standard uncertainties are shown in parentheses. b Standard deviation of the least-squares fit.
conclude that the barrier for collisional cooling of the second lowest energy conformer is lower than 400 cm−1 , allowing effectively complete cooling to a single conformational minimum to be achieved. As seen in Table 3, the experimental rotational constants show slightly better agreement with conformer 1 and 2 than with any of the other conformers. Full structural details of the two lowest energy geometries as determined from the highest level of theory, MP2/6-311G∗∗ , are given in Table 4, and Fig. 3 provides a side-by-side visual comparison. Note that the calculated bond lengths of the two conformers are almost identical. Significant structural differences are found for the 6 H1,C2,O1,P dihedral angle, which is calculated to be −40◦ and +28◦ for geometries 1 & 2, respectively.
The experimentally determined value for θ , the angle between the a- principal inertial axis and P–C1 bond, can also be compared to the values obtained from the ab initio calculations. The values of the angle θ as determined from the MP2/6-311G∗∗ basis set for the two lowest energy conformers are presented in Table 5. As seen in Table 5, the calculated values exhibit poor agreement with the experimental value of 62.70(4)◦ , which is not understood at this time. Additional isotopic substitution experiments are planned to resolve this discrepancy. Indeed, measurements of isotopic species will provide additional structural parameters, which can then be compared directly with the calculated values. Electric dipole moment components along the three principal axes were also calculated from the ab initio structures
TABLE 3 Computed Relative Energies (Pure Electronic and Electronic plus Zero-Point Vibrational Energy) and Rotational Constants of the Five Sarin Isomers at Three Different Levels of Theory
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Ab initio calculations are complicated on this molecule by the multiple conformations and the large numbers of electrons distributed over several nuclear centers,. However, the theory clearly suggests that two low-energy conformers exist. Further work is necessary to determine which conformer is observed in the microwave experiment. A more complete structural analysis is possible through a microwave study of isotopically substituted Sarin, which should be possible for mono-substituted 13 C and 18 O without the need for isotopic enrichment. Plans for this work are presently underway. DISCUSSION
FIG. 3. A comparison of the two lowest energy conformers as determined from ab initio calculations at the MP2/6-311G∗∗ level of theory. See Table 4 for exact bond lengths and angles.
and are also listed in Table 5. Experimentally, only qualitative dipole-moment information was obtained. By comparing the microwave power required to generate the π/2 pulse conditions necessary to optimize intensities for transitions associated with all three selection rules, one can qualitatively determine the relative ordering of the magnitudes of the three dipole moment components. Experiments find the π/2 pulse conditions to be approximately the same for all three transition types; i.e., µa ≈ µb ≈ µc . Comparing these measurements with similar data on other molecules studied (where the dipole moment components are known) allows the dipole moment components of Sarin to be estimated to be ≥1 D along each principal axis. These qualitative results are compared with the theoretical dipole moments in Table 5 and again lead to little insight into which low energy conformer has actually been observed. TABLE 5 Experimental Parameters (θ, the Angle between the Methyl Group and a-Principal Inertial Axis, and µx , the Dipole Moment along the x Axis) Compared to Those Determined for the Two Lowest Energy Conformers obtained via the MP2/6-311G∗∗ Level of Theory
† 1D = 3.335 639 × 10−30 C·m.
Microwave spectroscopy using a Stark cell has previously been considered as a detection method for chemical warfare agents (11, 12), but the authors were pessimistic about the use of the Stark cell spectrometer as an alarm device for chemicalwarfare agents due to the weak signal strength and the inconvenience of the low pressures and high Stark voltages required. In Stark cell experiments, low-pressure sample requirements make wall absorption a large problem. Second, the sample cell in the Stark spectrometer is at or near room temperature. Roomtemperature rotational spectra of molecules such as Sarin are extremely dense and blended, making unambiguous and reliable detection difficult to impossible. FTMW spectroscopy, on the other hand, uses a pulsed nozzle operating at up to 20 Hz to achieve <2 K rotational temperatures. Inert carrier gas seeded with the species to be studied is pulsed through the nozzle and into the evacuated chamber. Usually carrier gases such as argon or neon are used for these adiabatic expansions; however, it has been shown that air can be used to deliver rotational temperatures slightly warmer than those achieved with inert gases (3). In this study, a number of rotational transitions of Sarin have been identified, assigned, and cataloged into a database for use with a FTMW spectrometer. Further isotopic substitution work would be necessary for a complete structural and conformational analysis of Sarin, but this should perhaps be delayed until the origin of the missing second low-lying conformer is understood. Barriers for internal rotation about C–P bonds, such as that determined here, have not been extensively studied, but by analogy with internal rotation barriers about C–C, C–N, and C–O bonds, one might expect a large range of barrier heights. The magnitudes of the barriers will depend on both steric considerations and on the nature of the other bonds attached to the P atom. REFERENCES 1. “Chemical and Biological Terrorism: Research and Development to Improve Civilian Medical Response.” The National Academy Press, Washington, DC, 1999; available at http://www.nap.edu/html/terrorism. 2. T. J. Balle and W. H. Flygare, Rev. Sci. Instrum. 52 (1), 33–45 (1981). 3. U. Andresen, H. Driezler, U. Kretschmer, W. Stahl, and C. Thomsen, Fresenius J. Anal. Chem. 349, 272 (1994).
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4. F. J. Lovas, Pereyra, R. D. Suenram, G. T. Fraser, J.-U. Grabow, and A. R. Hight Walker, in “Proc 1994 U.S. EPA/A&WMA Intern Symp.Optical Sensors for Environmental and Chemical Process Monitoring, McLean, VA.” 5. R. D. Suenram, J.-U. Grabow, Andrei Zuban, and Igor Leonov, Rev. Sci. Instrum. 70 (4), 2127–2135 (1999). 6. J. Z. Gillies, C. W. Gillies, F. J. Lovas, K. Matsumura, R. D. Suenram, E. Kraka, and D. Cremer, J. Am. Chem. Soc. 113, 6408–6415 (1991). 7. R. S. Ruoff, T. D. Klots, T. Emilsson, and H. S. Gutowsky, J. Chem. Phys. 93, 3142 (1990). 8. J. Nakagawa, M. Imachi, and M. Hayashi, J. Mol. Struct. 112, 201 (1984); E. Hiroto, J. Phys. Chem. 83, 1457–1465 (1979); and recent measurements performed in our laboratory. 9. R. C. Woods, J. Mol. Spectrosc. 21, 4–24 (1966); A. Bauder and HS. H. G¨unthard, J. Mol. Spectrosc. 60, 290–311 (1976).
10. M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montogomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez, and J. A. Pople, Gaussian 94, Revision E-2, Gaussian, Inc., Pittsburgh, 1995. 11. Geo-Centers, Inc., “Theoretical Evaluation of Microwave Absorption for Chemical Agents Detection,” Report GC-TR-82-255 (Rmol report). 12. J. Friend and C. Gelman, “Preliminary Study of the Microwave Spectrum of GB for Possible Application to Detection,” Interim Report CRLR 475 (Project 4-08-06-015), 1956.
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Rotational Spectrum of Sarin1 A. R. Hight Walker,∗ R. D. Suenram,∗ Alan Samuels,† James Jensen,† Michael W. Ellzy,† J. Michael Lochner,† and Daniel Zeroka‡ ∗ Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899; †Edgewood Research, Development, and Engineering Center, Aberdeen Proving Grounds, Aberdeen, Maryland 21010-5423; and ‡Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015-3172 Received December 19, 2000; published online April 3, 2001
As part of an effort to examine the possibility of using molecular-beam Fourier-transform microwave spectroscopy to unambiguously detect and monitor chemical warfare agents, we report the first observation and assignment of the rotational spectrum of the nerve agent Sarin (GB) (Methylphosphonofluoridic acid 1-methyl-ethyl ester, CAS #107-44-8) at frequencies between 10 and 22 GHz. Only one of the two low-energy conformers of this organophosphorus compound (C4 H10 FO2 P) was observed in the rotationally cold (Trot < 2 K) molecular beam. The experimental asymmetric-rotor ground-state rotational constants of this conformer are A = 2874.0710(9) MHz, B = 1168.5776(4) MHz, C = 1056.3363(4) MHz (Type A standard uncertainties are given, i.e., 1 σ ), as obtained from a least-squares analysis of 74 a-, b-, and c-type rotational transitions. Several of the transitions are split into doublets due to the internal rotation of the methyl group attached to the phosphorus. The three-foldsymmetry barrier to internal rotation estimated from these splittings is 677.0(4) cm−1 . Ab initio electronic structure calculations using Hartree–Fock, density functional, and Moller–Plesset perturbation theories have also been made. The structure of the lowest-energy conformer determined from a structural optimization at the MP2/6-311G∗∗ level of theory is consistent with our C 2001 Academic Press experimental findings. °
a database of microwave absorption frequencies and detection limits and a set of validated procedures for qualitative and quantitative chemical analysis. A reduction in the size, complexity, and cost of the instrument would also be valuable. Sarin is a challenge to modern spectroscopic techniques, since it is a molecule with eight heavy atoms, several predicted lowenergy conformers, and a large-amplitude motion. Virtually all molecules that display biological activity exhibit several conformers and have large-amplitude motions. The present paper is thus also part of a larger NIST effort aimed ultimately at the microwave investigation of secondary (folding) structures of biomimetics in the gas phase. Here, as a small step along that path, the rotational spectrum of Sarin is observed and assigned to a near rigid asymmetric rotor associated with the lowest energy conformational isomer of the molecule. This conformer has C1 point-group symmetry. Small splittings of the rotational lines, attributed to methyl-top internal-rotation tunneling about the C– P bond, imply a barrier to internal rotation of 677.0(4) cm−1 .
INTRODUCTION
The microwave spectrum of Sarin (GB) (Methylphosphonofluoridic acid 1-methyl-ethyl ester, CAS # 107-44-8) is of interest as part of an effort to develop microwave-based sensors for the detection of chemical-warfare agents. It is desired that such a sensor provide rapid, sensitive, portable, and speciesspecific quantitative detection of chemical warfare agents for use in treaty verification, safety monitoring at disposal sites, counter terrorism, and troop protection. Current detection technologies include ion-mobility spectrometry, flame photometry, gas chromatography, mass spectrometry, infrared spectroscopy, wet-chemical methods, surface-acoustic-wave devices, colorchange chemistry, and enzyme-based sensors (1). Each of these methods suffers from deficiencies in such areas as cost, specificity, complexity, false-alarm rate, reliability, and size. Here, we demonstrate the capabilities of Fourier-transform microwave (FTMW) spectroscopy (2) to detect chemical-warfare agents by observing the rotational spectrum of the relatively high vapor pressure (0.4 kPa at 298 K) nerve agent, Sarin. The present paper is part of a broader effort to develop FTMW spectroscopy for the detection, quantification, and realtime monitoring of various trace chemical species in air (3–5). For FTMW spectroscopy to be competitive against other techniques for the detection of trace gases it is necessary to develop
EXPERIMENTAL
To safely perform the experiments, a mobile FTMW spectrometer was installed in a surety laboratory at the U.S. Army Edgewood Chemical and Biological Center (ECBC). The capabilities of the staff at ECBC to synthesize Sarin and ensure its safe use were critical for the success of these experiments. A detailed description of the mobile FTMW spectrometer used for this project has been reported previously (5). Briefly, a
1 Throughout the course of this manuscript, certain trade names are necessary to adequately describe the software of the equipment used. The use of these trade names should not be construed as an endorsement by NIST in any way.
77
0022-2852/01 $35.00 C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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pulsed-molecular-beam of inert carrier gas seeded with Sarin vapor is injected coaxially into a high-Q (50,000–100,000) Fabry– Perot microwave cavity. The rotational temperature of the expansion of typically <2 K allows only one conformer of Sarin to be appreciably populated in the molecular beam. The Fabry–Perot resonator is housed in a vacuum chamber pumped by a diffusion pump to a base pressure of approximately 13 µPa (10−7 Torr). Once the molecular beam travels to the center of the cavity, microwave radiation, which is tunable from 10 to 26.5 GHz, is pulsed into the cavity via an L-shaped antenna mounted in the center of one of the mirrors. If a rotational transition lies within the 500-kHz bandwidth of the microwave cavity, the molecules are coherently excited. The free-induction decay signal produced by this coherent excitation is collected at a matched antenna in the second mirror and monitored by a superheterodyne receiver. The Fourier transform of the free-induction decay gives a spectrum from which the frequency of the rotational transition can be determined. Frequency measurements are referenced to the 10 MHz Loran-C frequency, which itself is referenced to a Cs clock with a stated standard uncertainty of 1 part in 1012 . For the work being described here, a flow nozzle (6) mounted on the integral end flange-mirror of the Fabry–Perot cavity was used to inject the gas into the chamber. A stainless steel inlet line transported a Ne/He (80%/20%) stream through a diffusion tube to collect small amounts of Sarin vapor prior to exiting from the nozzle. The 200-µl Sarin sample was kept in an exhaust hood during the experiments and only the dilute gas stream was flowed to the pulsed-molecular-beam valve. Flow controllers were used to maintain a flow rate of 20 to 30 cc/min through the nozzle, with the excess flow being routed back into the hood through a second stainless steel line. The exhaust gas from the roughing pump was also vented into the hood. To guide the microwave experiment, Molecular Mechanics (MM+) structural predictions were performed using HyperChem Version 4.5. Several conformers were predicted to lie at low energies, although the B + C rotational constants for the different conformers were similar (within 20%). All of the structures were predicted to have a significant projection of the electric dipole moment vector (µ > 3.3 × 10−30 C m [>1 Debye]) along the three principal inertial axes. The spectrum of each conformer of Sarin was expected to be that of a near prolate rotor, with a-type, R-branch transitions spaced by approximately B + C. The mobile FTMW spectrometer is not equipped with Stark plates for static electric-field measurements, so the rotational assignments were made by pattern recognition and verified through fitting to an asymmetric rotor internal-rotation Hamiltonian. ROTATIONAL ASSIGNMENT
Automated broadband survey spectra for Sarin were recorded at a scan rate of approximately 1 GHz/h. The spectral regions searched were guided by the molecular mechanics predictions for the positions of the a-type R-branch (i.e., 1J = 1, 1Ka = 0)
FIG. 1. Broad (2-GHz) survey scan of Sarin vapor. Entire scan took less than 3 h.
clumps. A sample of one such survey spectrum is shown in Fig. 1 for frequencies between 12 GHz and 14 GHz. The lines in the figure have been assigned to the lowest energy rotational or conformational isomer of Sarin. Searches in regions where transitions from other low-energy conformers are predicted were unsuccessful, demonstrating that only one conformer is populated in the rotationally cold molecular beam. We note that previous studies of conformational relaxation in cold molecular beams suggest efficient cooling when the barrier between the higher and lower energy conformer is less than 400 cm−1 (7). For higher barriers, the relative populations of the conformers are expected to be unchanged from the room-temperature distribution. The transitions measured in the low-resolution survey spectrum were recorded at high resolution, and the line frequencies were fit to a standard asymmetric-rotor Hamiltonian. Transitions were observed arising from all three asymmetricrotor electric-dipole selection rules (a-, b-, and c-type). A number of the lines were partially or, in some cases, fully split into equal-intensity doublets, as pictured in Fig. 2. This splitting is attributed to internal-rotation tunneling arising from one of the methyl tops of the Sarin molecule. The methyl tops on the isopropyl group are sterically hindered and are thus not likely to give rise to splittings of this magnitude. In fact, previous studies have shown that the high barriers to internal rotation for methyl tops in isopropyl groups do not give rise to observable splittings at the FTMW resolution (8). The observed doublet transitions are thus assigned to the tunneling-split A and E symmetry components arising from the internal-rotation tunneling of the methyl top attached to the phosphorus atom. Once the assignments were complete, a final data fit was made to an internal-rotation centrifugal-distortion Hamiltonian using a program written by Woods (9). The barrier to internal rotation of the methyl top attached to the phosphorus atom was determined to be V3 = 677.0(4) cm−1 . In addition, Coriolis interactions between the rotational and internal rotation angular momenta
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TABLE 1 Observed Rotational Transitions for Sarin1
FIG. 2. Two rotational transitions that have been split into two components of A and E symmetry due to the internal rotation of the methyl group attached to the phosphorus.
allow the angle between the methyl-top axis and the a-principal inertial axis of the molecule to be determined as 62.70(4)◦ . The assignments and frequencies of the 52 transitions used in the final analysis are listed in Table 1. The resulting rotational constants and internal-rotation parameters are given in Table 2. THEORETICAL CALCULATIONS
Ten different Sarin trial geometries were optimized using the standard HF, DFT/B3LYP, and MP2 levels of theory with a 6-311G∗∗ basis set, as implemented in the Gaussian 94 electronic-structure program package (10). These 10 geometries, which differ by rotation about the P–O and Cisopropyl –O bonds, optimize to five unique conformers. We note that isoenergetic enantiomers of Sarin are possible via the interchange of the oxygen and fluorine positions on the chiral phosphorus. Table 3 lists the relative energies and rotational constants for the five isomers as determined by the three different calculations. Although the energy difference between the two lowest-energy conformers is small, approximately 0.2 kJ/mol, the conformer energy ordering is invariant with level of theory, with the HF and MP2 calculations closely agreeing. Note that the energy ordering is the same whether one takes the electronic energy, E el , or the vibrational-zero-point-corrected electronic energy, E el+zpe . Assuming thermal equilibrium and a 1 K molecular-beam conformational temperature gives a fractional population of less than 10−15 for the second lowest energy conformer. Previous studies (7) of conformational isomerization demonstrate minimal conformational cooling in supersonic expansions, except when the barrier to conformational relaxation is less than 400 cm−1 . From the experiments undertaken for this paper, which included spectral searches for a second conformer, we
1
The standard uncertainties on the transition frequencies are 2 kHz. These transitions are not included in the least-squares fit since they exhibit unresolved A/E torsional splittings. 2
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TABLE 2 Rotational Constants of Sarin (GB)
TABLE 4 Structural Comparison of the Two Lowest Energy Conformers: Bond Lengths and Angles Calculated at the MP2/6-311G∗∗ Level of Theory
a Type A standard uncertainties are shown in parentheses. b Standard deviation of the least-squares fit.
conclude that the barrier for collisional cooling of the second lowest energy conformer is lower than 400 cm−1 , allowing effectively complete cooling to a single conformational minimum to be achieved. As seen in Table 3, the experimental rotational constants show slightly better agreement with conformer 1 and 2 than with any of the other conformers. Full structural details of the two lowest energy geometries as determined from the highest level of theory, MP2/6-311G∗∗ , are given in Table 4, and Fig. 3 provides a side-by-side visual comparison. Note that the calculated bond lengths of the two conformers are almost identical. Significant structural differences are found for the 6 H1,C2,O1,P dihedral angle, which is calculated to be −40◦ and +28◦ for geometries 1 & 2, respectively.
The experimentally determined value for θ , the angle between the a- principal inertial axis and P–C1 bond, can also be compared to the values obtained from the ab initio calculations. The values of the angle θ as determined from the MP2/6-311G∗∗ basis set for the two lowest energy conformers are presented in Table 5. As seen in Table 5, the calculated values exhibit poor agreement with the experimental value of 62.70(4)◦ , which is not understood at this time. Additional isotopic substitution experiments are planned to resolve this discrepancy. Indeed, measurements of isotopic species will provide additional structural parameters, which can then be compared directly with the calculated values. Electric dipole moment components along the three principal axes were also calculated from the ab initio structures
TABLE 3 Computed Relative Energies (Pure Electronic and Electronic plus Zero-Point Vibrational Energy) and Rotational Constants of the Five Sarin Isomers at Three Different Levels of Theory
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ROTATIONAL SPECTRUM OF SARIN
Ab initio calculations are complicated on this molecule by the multiple conformations and the large numbers of electrons distributed over several nuclear centers,. However, the theory clearly suggests that two low-energy conformers exist. Further work is necessary to determine which conformer is observed in the microwave experiment. A more complete structural analysis is possible through a microwave study of isotopically substituted Sarin, which should be possible for mono-substituted 13 C and 18 O without the need for isotopic enrichment. Plans for this work are presently underway. DISCUSSION
FIG. 3. A comparison of the two lowest energy conformers as determined from ab initio calculations at the MP2/6-311G∗∗ level of theory. See Table 4 for exact bond lengths and angles.
and are also listed in Table 5. Experimentally, only qualitative dipole-moment information was obtained. By comparing the microwave power required to generate the π/2 pulse conditions necessary to optimize intensities for transitions associated with all three selection rules, one can qualitatively determine the relative ordering of the magnitudes of the three dipole moment components. Experiments find the π/2 pulse conditions to be approximately the same for all three transition types; i.e., µa ≈ µb ≈ µc . Comparing these measurements with similar data on other molecules studied (where the dipole moment components are known) allows the dipole moment components of Sarin to be estimated to be ≥1 D along each principal axis. These qualitative results are compared with the theoretical dipole moments in Table 5 and again lead to little insight into which low energy conformer has actually been observed. TABLE 5 Experimental Parameters (θ, the Angle between the Methyl Group and a-Principal Inertial Axis, and µx , the Dipole Moment along the x Axis) Compared to Those Determined for the Two Lowest Energy Conformers obtained via the MP2/6-311G∗∗ Level of Theory
† 1D = 3.335 639 × 10−30 C·m.
Microwave spectroscopy using a Stark cell has previously been considered as a detection method for chemical warfare agents (11, 12), but the authors were pessimistic about the use of the Stark cell spectrometer as an alarm device for chemicalwarfare agents due to the weak signal strength and the inconvenience of the low pressures and high Stark voltages required. In Stark cell experiments, low-pressure sample requirements make wall absorption a large problem. Second, the sample cell in the Stark spectrometer is at or near room temperature. Roomtemperature rotational spectra of molecules such as Sarin are extremely dense and blended, making unambiguous and reliable detection difficult to impossible. FTMW spectroscopy, on the other hand, uses a pulsed nozzle operating at up to 20 Hz to achieve <2 K rotational temperatures. Inert carrier gas seeded with the species to be studied is pulsed through the nozzle and into the evacuated chamber. Usually carrier gases such as argon or neon are used for these adiabatic expansions; however, it has been shown that air can be used to deliver rotational temperatures slightly warmer than those achieved with inert gases (3). In this study, a number of rotational transitions of Sarin have been identified, assigned, and cataloged into a database for use with a FTMW spectrometer. Further isotopic substitution work would be necessary for a complete structural and conformational analysis of Sarin, but this should perhaps be delayed until the origin of the missing second low-lying conformer is understood. Barriers for internal rotation about C–P bonds, such as that determined here, have not been extensively studied, but by analogy with internal rotation barriers about C–C, C–N, and C–O bonds, one might expect a large range of barrier heights. The magnitudes of the barriers will depend on both steric considerations and on the nature of the other bonds attached to the P atom. REFERENCES 1. “Chemical and Biological Terrorism: Research and Development to Improve Civilian Medical Response.” The National Academy Press, Washington, DC, 1999; available at http://www.nap.edu/html/terrorism. 2. T. J. Balle and W. H. Flygare, Rev. Sci. Instrum. 52 (1), 33–45 (1981). 3. U. Andresen, H. Driezler, U. Kretschmer, W. Stahl, and C. Thomsen, Fresenius J. Anal. Chem. 349, 272 (1994).
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4. F. J. Lovas, Pereyra, R. D. Suenram, G. T. Fraser, J.-U. Grabow, and A. R. Hight Walker, in “Proc 1994 U.S. EPA/A&WMA Intern Symp.Optical Sensors for Environmental and Chemical Process Monitoring, McLean, VA.” 5. R. D. Suenram, J.-U. Grabow, Andrei Zuban, and Igor Leonov, Rev. Sci. Instrum. 70 (4), 2127–2135 (1999). 6. J. Z. Gillies, C. W. Gillies, F. J. Lovas, K. Matsumura, R. D. Suenram, E. Kraka, and D. Cremer, J. Am. Chem. Soc. 113, 6408–6415 (1991). 7. R. S. Ruoff, T. D. Klots, T. Emilsson, and H. S. Gutowsky, J. Chem. Phys. 93, 3142 (1990). 8. J. Nakagawa, M. Imachi, and M. Hayashi, J. Mol. Struct. 112, 201 (1984); E. Hiroto, J. Phys. Chem. 83, 1457–1465 (1979); and recent measurements performed in our laboratory. 9. R. C. Woods, J. Mol. Spectrosc. 21, 4–24 (1966); A. Bauder and HS. H. G¨unthard, J. Mol. Spectrosc. 60, 290–311 (1976).
10. M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montogomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez, and J. A. Pople, Gaussian 94, Revision E-2, Gaussian, Inc., Pittsburgh, 1995. 11. Geo-Centers, Inc., “Theoretical Evaluation of Microwave Absorption for Chemical Agents Detection,” Report GC-TR-82-255 (Rmol report). 12. J. Friend and C. Gelman, “Preliminary Study of the Microwave Spectrum of GB for Possible Application to Detection,” Interim Report CRLR 475 (Project 4-08-06-015), 1956.
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