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Fourier Transform Microwave Spectroscopy of AlBr
Journal of Molecular Spectroscopy 193, 224 –227 (1999) Article ID jmsp.1998.7720, available online at http://www.idealibrary.com on
NOTE Fourier Transform Microwave Spectroscopy of AlBr Pure rotational transitions of aluminum monobromide were first observed by Wyse and Gordy (1) using millimeter-wave spectroscopy. They measured transitions for both the Al79Br and Al81Br isotopomers in several vibrational states (v 5 0 –5) and calculated an equilibrium (r e) bond distance. More recently, Uehara et al. (2) used a Fourier transform infrared spectrometer to measure the ro–vibrational spectrum of aluminum monobromide in emission. They combined their results with those of the millimeter-wave study to determine Dunham potential coefficients. There has been only one study of the hyperfine structure of AlBr. The nuclear quadrupole structure of the J 5 1–0 transition of Al79Br, but not Al81Br, was measured by Hoeft et al. (3). Nuclear quadrupole hyperfine splitting was not observed in the millimeter-wave study (1) because the measured transitions were at relatively high J. The present work was undertaken to investigate further the hyperfine structure of AlBr. The hyperfine splittings of the J 5 1–0 and J 5 2–1 pure rotational transitions of both isotopomers of aluminum monobromide, Al79Br and Al81Br, have been observed in both the ground and first-excited vibrational states. These measurements were made using a Balle–Flygare-type Fourier transform microwave (FTMW) spectrometer (4) with a laser ablation source (5). From these results, the nuclear quadrupole coupling constants for aluminum and for both bromine nuclei have been calculated and, for the first time, the nuclear spin–rotation and nuclear spin–spin constants have been determined. AlBr samples were produced by reacting ablated aluminum (rod from Alfa AESAR 99.999%) with bromine, which was present as 0.05– 0.1% in Ar backing gas. Laser energies of approximately 5–10 mJ/pulse from a frequencydoubled Nd:YAG laser were used and the pulsed nozzle was operated at a backing pressure of 6 atm. The strongest transitions could be seen easily in a few averaging cycles. Two thousand averaging cycles were accumulated for the weakest transitions to obtain reasonable signal-to-noise ratios. An example transition is shown in Fig. 1. Each hyperfine component appears as a doublet
FIG. 1. Part of the J 5 1–0 F 1 5 23 – 23 transition of Al81Br. This spectrum was obtained with 400 averaging cycles. The microwave excitation frequency was 9473.885 MHz. Data points totalling 4 K were measured with 50-ns sampling time and the power spectrum is displayed as an 8 K transformation. The hyperfine components were assigned using the coupling scheme J 1 IBr 5 F 1 ; F1 1 IAl 5 F.
due to the Doppler effect because the molecular jet is injected into the cavity parallel to the axis of microwave propagation. The line positions of unblended lines were determined by averaging the frequencies of the Doppler components obtained from the power spectrum. Frequencies of closely spaced or overlapped lines were determined by fitting to their time domain signals (6). Since the rotational and centrifugal distortion constants of both isotopomers of AlBr were known from the millimeter-wave study (1), the initial search range was quite narrow. Only the two lowest J rotational transitions were available in the frequency range of the spectrometer. Nuclear quadrupole interactions due to both Br (I 5 23 for both isotopes, 79Br and 81Br) and 27Al (I 5 25 ) were included in the prediction using the 27Al and 79Br nuclear quadrupole coupling constants reported for Al79Br by Hoeft et al. (3) and a 81 Br nuclear quadrupole coupling constant obtained by scaling eQq (79Br) by the ratio of nuclear quadrupole moments. The observed transitions of Al79Br and Al81Br in the v 5 0 and v 5 1 vibrational states are listed in Tables 1 and 2, respectively. The hyperfine components for each isotopomer were assigned using the coupling scheme J 1 IBr 5 F1; F1 1 IAl 5 F. An overview spectrum of the J 5 1–0 transition of Al79Br is shown in Fig. 2. The transitions for each isotopomer in each vibrational state were fit separately using Pickett’s exact fitting program SPFIT (7) to determine the rotational and centrifugal distortion constants, B and D, as well as the nuclear spin–spin constant, aAl–Br, the nuclear quadrupole coupling constants, eQq,
FIG. 2. Composite spectrum of the J 5 1–0 rotational transition of Al79Br. The transition is split into three groups by the 79Br nucleus and the further splitting within each group is due to the 27Al nucleus. The results of twelve different microwave experiments were used to produce this composite. The individual power spectra were scaled to reproduce predicted intensities.
224 0022-2852/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
225
NOTE
TABLE 1 Measured Frequencies of J 5 1– 0 and J 5 2–1 Transitions of Al79Br in v 5 0 and v 5 1 Vibrational Statesa
Measurement accuracy is estimated to be better than 61 kHz. Microwave synthesizer is referenced to Loran frequency standard, which is accurate to 1 part in 1012. a
and nuclear spin–rotation constants, C I , for both the aluminum and bromine nuclei. The constants determined for Al79Br and Al81Br are listed in Tables 3 and 4, respectively. Also listed are literature values for the rotational and centrifugal distortion constants (1) and the 27Al and 79Br nuclear quadrupole coupling constants for Al79Br (3). These constants agree quite well. No attempt was made to do a global fit including the millimeter-wave results (1) with the present data. The former study covered a wide range of frequencies, J values and vibrational states, and was used to determine Dunham coefficients. These coefficients give excellent centrifugal distortion constants, to which the present data can add nothing. The uncertainties in the present rotational constants are only a factor of 3 better than those from the Dunham coefficients. On the other hand, the Al79Br nuclear quadrupole coupling constants determined in our study are two orders of magnitude more precise than those obtained by Hoeft et al. (3). The improved precision of the nuclear quadrupole coupling constants and the determination of the nuclear spin–rotation constants and nuclear spin–spin constant demonstrate once again the utility of FTMW spectroscopy technique for determining small hyperfine parameters. The nuclear spin–spin constant, aAl–Br, should be consistent with the inter-
nuclear bond length. Assuming that the nuclear spin–spin interaction is due only to direct contributions, aAl–Br can be expressed as (8)
a Al–Br 5
23 m 2Ng Alg Br , 3 r Al–Br
[1]
where m N is the nuclear magneton and g Al and g Br are the nuclear g factors for the Al and Br nuclei. Internuclear bond distances of 2.31(13) and 2.47(17) Å were calculated from the nuclear spin–spin constants of Al79Br and Al81Br, respectively. The large uncertainties in the estimated bond distances are due to uncertainties in the respective constants. Despite the large uncertainties, the estimated internuclear distances agree with the r e value of 2.29480(3) Å (1).
ACKNOWLEDGMENTS Support for this research has been provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Petroleum Research Fund administered by the American Chemical Society.
Copyright © 1999 by Academic Press
226
NOTE
TABLE 2 Measured Frequencies of J 5 1– 0 and J 5 2–1 Transitions of Al81Br in v 5 0 and v 5 1 Vibrational Statesa
a Measurement accuracy is estimated to be better than 61 kHz. Microwave syntheszier is referenced to Loran frequency standard, which is accurate to 1 part in 1012.
TABLE 3 Molecular Constants Calculated for Al79Br in MHza
a
One standard deviation in parentheses, in units of least significant digit. Calculated from results of Wyse and Gordy [1]. c Results taken from Hoeft et al. [3]. d Nuclear spin-spin constant held fixed at value obtained from v 5 0 fit. b
Copyright © 1999 by Academic Press
227
NOTE
TABLE 4 Molecular Constants Calculated for Al81Br in MHza
a
One standard deviation in parentheses, in units of least significant digit. Calculated from results of Wyse and Gordy [1]. c Nuclear spin-spin constant held fixed at value obtained from v 5 0 fit. b
REFERENCES
6. J. Haekel and H. Ma¨der, Z. Naturforsch. A 43, 203–206 (1988). 7. H. M. Pickett, J. Mol. Spectrosc. 148, 371–377 (1991). 8. C. Styger and M. C. L. Gerry, J. Mol. Spectrosc. 158, 328 –338 (1993).
1. F. C. Wyse and W. Gordy, J. Chem. Phys. 56, 2130 –2136 (1972). 2. H. Uehara, K. Horiai, Y. Ozaki, and T. Konno, Chem. Phys. Lett. 214, 527–530 (1993). 3. J. Hoeft, T. To¨rring, and E. Tiemann, Z. Naturforsch. A 28, 1066 –1068 (1973). 4. Y. Xu, W. Ja¨ger, and M. C. L. Gerry, J. Mol. Spectrosc. 151, 206 –216 (1992). 5. K. A. Walker and M. C. L. Gerry, J. Mol. Spectrosc. 182, 178 –183 (1997).
Kaley A. Walker Michael C. L. Gerry Department of Chemistry The University of British Columbia 2036 Main Mall, Vancouver, British Columbia Canada V6T1Z1 Received May 8, 1998; in revised form May 26, 1998
Copyright © 1999 by Academic Press
NOTE Fourier Transform Microwave Spectroscopy of AlBr Pure rotational transitions of aluminum monobromide were first observed by Wyse and Gordy (1) using millimeter-wave spectroscopy. They measured transitions for both the Al79Br and Al81Br isotopomers in several vibrational states (v 5 0 –5) and calculated an equilibrium (r e) bond distance. More recently, Uehara et al. (2) used a Fourier transform infrared spectrometer to measure the ro–vibrational spectrum of aluminum monobromide in emission. They combined their results with those of the millimeter-wave study to determine Dunham potential coefficients. There has been only one study of the hyperfine structure of AlBr. The nuclear quadrupole structure of the J 5 1–0 transition of Al79Br, but not Al81Br, was measured by Hoeft et al. (3). Nuclear quadrupole hyperfine splitting was not observed in the millimeter-wave study (1) because the measured transitions were at relatively high J. The present work was undertaken to investigate further the hyperfine structure of AlBr. The hyperfine splittings of the J 5 1–0 and J 5 2–1 pure rotational transitions of both isotopomers of aluminum monobromide, Al79Br and Al81Br, have been observed in both the ground and first-excited vibrational states. These measurements were made using a Balle–Flygare-type Fourier transform microwave (FTMW) spectrometer (4) with a laser ablation source (5). From these results, the nuclear quadrupole coupling constants for aluminum and for both bromine nuclei have been calculated and, for the first time, the nuclear spin–rotation and nuclear spin–spin constants have been determined. AlBr samples were produced by reacting ablated aluminum (rod from Alfa AESAR 99.999%) with bromine, which was present as 0.05– 0.1% in Ar backing gas. Laser energies of approximately 5–10 mJ/pulse from a frequencydoubled Nd:YAG laser were used and the pulsed nozzle was operated at a backing pressure of 6 atm. The strongest transitions could be seen easily in a few averaging cycles. Two thousand averaging cycles were accumulated for the weakest transitions to obtain reasonable signal-to-noise ratios. An example transition is shown in Fig. 1. Each hyperfine component appears as a doublet
FIG. 1. Part of the J 5 1–0 F 1 5 23 – 23 transition of Al81Br. This spectrum was obtained with 400 averaging cycles. The microwave excitation frequency was 9473.885 MHz. Data points totalling 4 K were measured with 50-ns sampling time and the power spectrum is displayed as an 8 K transformation. The hyperfine components were assigned using the coupling scheme J 1 IBr 5 F 1 ; F1 1 IAl 5 F.
due to the Doppler effect because the molecular jet is injected into the cavity parallel to the axis of microwave propagation. The line positions of unblended lines were determined by averaging the frequencies of the Doppler components obtained from the power spectrum. Frequencies of closely spaced or overlapped lines were determined by fitting to their time domain signals (6). Since the rotational and centrifugal distortion constants of both isotopomers of AlBr were known from the millimeter-wave study (1), the initial search range was quite narrow. Only the two lowest J rotational transitions were available in the frequency range of the spectrometer. Nuclear quadrupole interactions due to both Br (I 5 23 for both isotopes, 79Br and 81Br) and 27Al (I 5 25 ) were included in the prediction using the 27Al and 79Br nuclear quadrupole coupling constants reported for Al79Br by Hoeft et al. (3) and a 81 Br nuclear quadrupole coupling constant obtained by scaling eQq (79Br) by the ratio of nuclear quadrupole moments. The observed transitions of Al79Br and Al81Br in the v 5 0 and v 5 1 vibrational states are listed in Tables 1 and 2, respectively. The hyperfine components for each isotopomer were assigned using the coupling scheme J 1 IBr 5 F1; F1 1 IAl 5 F. An overview spectrum of the J 5 1–0 transition of Al79Br is shown in Fig. 2. The transitions for each isotopomer in each vibrational state were fit separately using Pickett’s exact fitting program SPFIT (7) to determine the rotational and centrifugal distortion constants, B and D, as well as the nuclear spin–spin constant, aAl–Br, the nuclear quadrupole coupling constants, eQq,
FIG. 2. Composite spectrum of the J 5 1–0 rotational transition of Al79Br. The transition is split into three groups by the 79Br nucleus and the further splitting within each group is due to the 27Al nucleus. The results of twelve different microwave experiments were used to produce this composite. The individual power spectra were scaled to reproduce predicted intensities.
224 0022-2852/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
225
NOTE
TABLE 1 Measured Frequencies of J 5 1– 0 and J 5 2–1 Transitions of Al79Br in v 5 0 and v 5 1 Vibrational Statesa
Measurement accuracy is estimated to be better than 61 kHz. Microwave synthesizer is referenced to Loran frequency standard, which is accurate to 1 part in 1012. a
and nuclear spin–rotation constants, C I , for both the aluminum and bromine nuclei. The constants determined for Al79Br and Al81Br are listed in Tables 3 and 4, respectively. Also listed are literature values for the rotational and centrifugal distortion constants (1) and the 27Al and 79Br nuclear quadrupole coupling constants for Al79Br (3). These constants agree quite well. No attempt was made to do a global fit including the millimeter-wave results (1) with the present data. The former study covered a wide range of frequencies, J values and vibrational states, and was used to determine Dunham coefficients. These coefficients give excellent centrifugal distortion constants, to which the present data can add nothing. The uncertainties in the present rotational constants are only a factor of 3 better than those from the Dunham coefficients. On the other hand, the Al79Br nuclear quadrupole coupling constants determined in our study are two orders of magnitude more precise than those obtained by Hoeft et al. (3). The improved precision of the nuclear quadrupole coupling constants and the determination of the nuclear spin–rotation constants and nuclear spin–spin constant demonstrate once again the utility of FTMW spectroscopy technique for determining small hyperfine parameters. The nuclear spin–spin constant, aAl–Br, should be consistent with the inter-
nuclear bond length. Assuming that the nuclear spin–spin interaction is due only to direct contributions, aAl–Br can be expressed as (8)
a Al–Br 5
23 m 2Ng Alg Br , 3 r Al–Br
[1]
where m N is the nuclear magneton and g Al and g Br are the nuclear g factors for the Al and Br nuclei. Internuclear bond distances of 2.31(13) and 2.47(17) Å were calculated from the nuclear spin–spin constants of Al79Br and Al81Br, respectively. The large uncertainties in the estimated bond distances are due to uncertainties in the respective constants. Despite the large uncertainties, the estimated internuclear distances agree with the r e value of 2.29480(3) Å (1).
ACKNOWLEDGMENTS Support for this research has been provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Petroleum Research Fund administered by the American Chemical Society.
Copyright © 1999 by Academic Press
226
NOTE
TABLE 2 Measured Frequencies of J 5 1– 0 and J 5 2–1 Transitions of Al81Br in v 5 0 and v 5 1 Vibrational Statesa
a Measurement accuracy is estimated to be better than 61 kHz. Microwave syntheszier is referenced to Loran frequency standard, which is accurate to 1 part in 1012.
TABLE 3 Molecular Constants Calculated for Al79Br in MHza
a
One standard deviation in parentheses, in units of least significant digit. Calculated from results of Wyse and Gordy [1]. c Results taken from Hoeft et al. [3]. d Nuclear spin-spin constant held fixed at value obtained from v 5 0 fit. b
Copyright © 1999 by Academic Press
227
NOTE
TABLE 4 Molecular Constants Calculated for Al81Br in MHza
a
One standard deviation in parentheses, in units of least significant digit. Calculated from results of Wyse and Gordy [1]. c Nuclear spin-spin constant held fixed at value obtained from v 5 0 fit. b
REFERENCES
6. J. Haekel and H. Ma¨der, Z. Naturforsch. A 43, 203–206 (1988). 7. H. M. Pickett, J. Mol. Spectrosc. 148, 371–377 (1991). 8. C. Styger and M. C. L. Gerry, J. Mol. Spectrosc. 158, 328 –338 (1993).
1. F. C. Wyse and W. Gordy, J. Chem. Phys. 56, 2130 –2136 (1972). 2. H. Uehara, K. Horiai, Y. Ozaki, and T. Konno, Chem. Phys. Lett. 214, 527–530 (1993). 3. J. Hoeft, T. To¨rring, and E. Tiemann, Z. Naturforsch. A 28, 1066 –1068 (1973). 4. Y. Xu, W. Ja¨ger, and M. C. L. Gerry, J. Mol. Spectrosc. 151, 206 –216 (1992). 5. K. A. Walker and M. C. L. Gerry, J. Mol. Spectrosc. 182, 178 –183 (1997).
Kaley A. Walker Michael C. L. Gerry Department of Chemistry The University of British Columbia 2036 Main Mall, Vancouver, British Columbia Canada V6T1Z1 Received May 8, 1998; in revised form May 26, 1998
Copyright © 1999 by Academic Press