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Isotopic splitting in matrix isolated BF3
Volume 9. number 3
CHEMICAL
ISOTOPIC
SPLITTING
1 May 1971
PHYSICS LETTERS
IN MATRIX LW.LEVIN
BF3
ISOLATED .
National Institute? of Arfhrttis and Metabclic Dtseases, NationnI lnstilutos of Heafih, Bethesda, hiarylard 80014,
USA
and S. ABRAMOWM’Z National Bureau of Standards, Received
12
Washington,
D. C 20014, USA
March 1971
Small isotopic fret;uency shift information, if precisely determined, provides an effective constraint on intramolecular force fields. The most precise data for these frequency shift parameters AU are derived from high resolution. gas-phase infrared spectral analyses. In the present study, we compare fsr BF the isotopic frequeooy shift Au from a recently published gas-phase study with the value for Au4 tha?we obtain from the spectrum o 4 matrix isolated BF3. The excellent agreement between the hvo methods suggests further apptications of the matrlK technique for obtaining precise frequency shift data.
Ginn et al. [lf recently discussed the BP3 E’ species force field in which the Coriolis zeta constants, lo<,, and 11~~~~ and the isotopic
frequency shift Av4 were used to fix the F34 interaction force constant. The smaf1 isotopic frequency shtit parameter LIv4 served as the most effective constraint primarily as a consequence of both its sensitivity to the force field und its highly precise value (Au4 = 1.77 f 0.02 cm-l). The gas-phase ~4 fundamental frequencies used to determine Au4 were calculated from the ob-
served band constant derived from an analysis of high resolution infrared spectra (0.03 cm-l) of isotopi~ally enriched samples [ 1J, Since this detailed approach is obviously impractical for many gas-phase systems, low temperature matrix studies may provide small isotopic frequency shifts with the required precision as an aiternative to the involved high resolution measurements and anaIyses [Z]. Although the vibrationrotation spectra are enormously simplified in an inert matrix, a question remains concerning the extent, if any, to which the matrix environment perturbs the observed isotopic splitting. In order to compare more closely the spiittings determined from gas-phase and matrix measurements aud, particularly, in light of the current high resolution studies of BF:, [l, 31, we repotit the ’ results of an earlier low temperature study in’ which we,examined ,&t iiqtid hydrogen temperatures the matrix isolated infrared spectra of the v4 mode of SQ. :
_),
Commercial samples of BF3 as well as isotoxically enriched samples were used_ ‘Mrared
spectra were obtained with a Perkin-Ehner model If2 G monochromator aE a spectral sIit width of about 0.5 cm-l *. An Air Products Cryotip * supplied the refrigeration at liquid hydrogen temperatures. Samples with argon as the matrix
diluent (1 : 500) were deposited on a CsK window at various flow rates. A typical deposition placed about ‘70 gmoles of sample on the window over a two hour period. internal and external absorption
standards were used to calibrate the observed spectra A representative scan of an enriched sample appears in fig. 1. An average of five expanded, slow scans gives a matrix isotopic frequency shift for &V4 of 1.9 f 0.1 cm-l, in good agreement with the gas-phase value of 1.77 cm-l_ Although isotopically enriched sampl6s were used in the matrix experiment, the overlapping 10~ and IlB contours. for ~4 limit the pre$sion of the frequencydifference to about 0.1 cm
. It.is of interest to compare the sets oEforce
constants using both the matrix and gas-phase isotopic shifts. As in the study of Ginn et al [lf, we con$%ucted + pbt of Au4 as 2 function of F34. Although g&phase frequencies were used to de* Q&ain cornme!&&instrumentsaxe identified to
~8pocifycomptetely, the experifoenht procedure, In no case does such fdentification impIy a recommendation or endorsement by the NationzE B&au of Standards r&&e National Insti,Wes of Heafth. ,I’,)
:
.,
247
Volume 9,
number 3
CKEMICAL PHYSICSLETTIhS’
Fig. 1.
I May 1971
Infrared spectrum of matrix isolated BF3. Table 1
Observed isotopic frequency shifta (cm-l)
and force constants (mdyn/l() for the E’ species Of BF3
Frequency shift, Av4 I.9 * 0.1 cm 1.77 + 0.02 t&-l
(matrix) (gas-phase)
F34
F33
a)
F44
6.68 f 0.12
-0.38
f 0.04
0.51
6.515 * 0.02
-0.329
* 0.01
0.5164 k 0.001
k 0.01
a) Ref. iI].
termine Av4, we use the observed matrix isotopic shift to fix the interaction force c.?nstant as~follo~s: F34 = -0.38 f 0.04 mdynes/A. Table
1 summarizes the potential fanction data for both the mat& and gas-phase shifts. As indicated in
the table, the potential constants resulting from the gas-phase shift and the matrix shift are in reasonable agreement. The force field from the matrix dsta, however, agrees extremely well with the individual potentsal functions that Ginn et 21. [l] obtained with lo<44 and 11C44, the isotopii: zeta constants, as constraints; namely,
F34 = -0.325 and F34 = -0.384, respectively. TM agreement may be Fortuitous as a result of ~e-s~~ic~tly larger dispersion in the A”4
value. frequencies for the v4 modes are blue ‘shifted slightly ‘in *+e matrix; thal is, the gasphase fre?,uencles at 481.13 [l] (!%F9) and 479.36 cm-1 [I] (IlBF ) increase in the matrix to 482.7 md 480.8 cm*\, -respectively. Since ‘iviewould expect the matrix to perk-b the ~4 modes I ‘, : ‘I, _’ ,. _‘matrix
The
: ‘-id8
: .
:
_., ‘_,
,;,
-.
,.
”
of both isotopes equally, the slightly kirger value for Av4 in the matrix over that obtained in the gas-phase (1.9 to 1.77 cm-l) may again reflect the overlapping of the contours of the two iso-, tapes. In summary, Av4 for BF3 determined from the infrared matrix spectra agrees quite well with the gas-phase frequency shift obtained from high resoktion studies. This encour2ging result reinforces the general suggestion concerning the applicability of matrix information toward determining precise frequency shift parameters for use in constraining intramolecular force fields. REFERENCES 111 S. G. W. Giun, D. Johansen and J. Overend, J. Mol. Spectry. 36 (1970) 448. [Z] A. A. Chalmers and D. C.McKean, S&%3x~hidb. Acta 22 (1966) 251. [3] S. G. W. Ginn,. C. WI Brow& J.5. Kcinnegand J. Cverend, J.Mol. 3pcctry. 28 4968} SOS. ;’
‘-
., .‘. .‘-. :, ,.,,_ ’ : .‘, ,, .‘. ,’ (.” ‘1.. ., ; ,” : .’ _. ,’ . .I.: (.._. ., ,..‘( -. _, ..-, :_ :_ ,; .,. t,i ., ,,,,,./’ ,,’ .-,’ ‘..,I :_. (’ .. . :. ‘, :
CHEMICAL
ISOTOPIC
SPLITTING
1 May 1971
PHYSICS LETTERS
IN MATRIX LW.LEVIN
BF3
ISOLATED .
National Institute? of Arfhrttis and Metabclic Dtseases, NationnI lnstilutos of Heafih, Bethesda, hiarylard 80014,
USA
and S. ABRAMOWM’Z National Bureau of Standards, Received
12
Washington,
D. C 20014, USA
March 1971
Small isotopic fret;uency shift information, if precisely determined, provides an effective constraint on intramolecular force fields. The most precise data for these frequency shift parameters AU are derived from high resolution. gas-phase infrared spectral analyses. In the present study, we compare fsr BF the isotopic frequeooy shift Au from a recently published gas-phase study with the value for Au4 tha?we obtain from the spectrum o 4 matrix isolated BF3. The excellent agreement between the hvo methods suggests further apptications of the matrlK technique for obtaining precise frequency shift data.
Ginn et al. [lf recently discussed the BP3 E’ species force field in which the Coriolis zeta constants, lo<,, and 11~~~~ and the isotopic
frequency shift Av4 were used to fix the F34 interaction force constant. The smaf1 isotopic frequency shtit parameter LIv4 served as the most effective constraint primarily as a consequence of both its sensitivity to the force field und its highly precise value (Au4 = 1.77 f 0.02 cm-l). The gas-phase ~4 fundamental frequencies used to determine Au4 were calculated from the ob-
served band constant derived from an analysis of high resolution infrared spectra (0.03 cm-l) of isotopi~ally enriched samples [ 1J, Since this detailed approach is obviously impractical for many gas-phase systems, low temperature matrix studies may provide small isotopic frequency shifts with the required precision as an aiternative to the involved high resolution measurements and anaIyses [Z]. Although the vibrationrotation spectra are enormously simplified in an inert matrix, a question remains concerning the extent, if any, to which the matrix environment perturbs the observed isotopic splitting. In order to compare more closely the spiittings determined from gas-phase and matrix measurements aud, particularly, in light of the current high resolution studies of BF:, [l, 31, we repotit the ’ results of an earlier low temperature study in’ which we,examined ,&t iiqtid hydrogen temperatures the matrix isolated infrared spectra of the v4 mode of SQ. :
_),
Commercial samples of BF3 as well as isotoxically enriched samples were used_ ‘Mrared
spectra were obtained with a Perkin-Ehner model If2 G monochromator aE a spectral sIit width of about 0.5 cm-l *. An Air Products Cryotip * supplied the refrigeration at liquid hydrogen temperatures. Samples with argon as the matrix
diluent (1 : 500) were deposited on a CsK window at various flow rates. A typical deposition placed about ‘70 gmoles of sample on the window over a two hour period. internal and external absorption
standards were used to calibrate the observed spectra A representative scan of an enriched sample appears in fig. 1. An average of five expanded, slow scans gives a matrix isotopic frequency shift for &V4 of 1.9 f 0.1 cm-l, in good agreement with the gas-phase value of 1.77 cm-l_ Although isotopically enriched sampl6s were used in the matrix experiment, the overlapping 10~ and IlB contours. for ~4 limit the pre$sion of the frequencydifference to about 0.1 cm
. It.is of interest to compare the sets oEforce
constants using both the matrix and gas-phase isotopic shifts. As in the study of Ginn et al [lf, we con$%ucted + pbt of Au4 as 2 function of F34. Although g&phase frequencies were used to de* Q&ain cornme!&&instrumentsaxe identified to
~8pocifycomptetely, the experifoenht procedure, In no case does such fdentification impIy a recommendation or endorsement by the NationzE B&au of Standards r&&e National Insti,Wes of Heafth. ,I’,)
:
.,
247
Volume 9,
number 3
CKEMICAL PHYSICSLETTIhS’
Fig. 1.
I May 1971
Infrared spectrum of matrix isolated BF3. Table 1
Observed isotopic frequency shifta (cm-l)
and force constants (mdyn/l() for the E’ species Of BF3
Frequency shift, Av4 I.9 * 0.1 cm 1.77 + 0.02 t&-l
(matrix) (gas-phase)
F34
F33
a)
F44
6.68 f 0.12
-0.38
f 0.04
0.51
6.515 * 0.02
-0.329
* 0.01
0.5164 k 0.001
k 0.01
a) Ref. iI].
termine Av4, we use the observed matrix isotopic shift to fix the interaction force c.?nstant as~follo~s: F34 = -0.38 f 0.04 mdynes/A. Table
1 summarizes the potential fanction data for both the mat& and gas-phase shifts. As indicated in
the table, the potential constants resulting from the gas-phase shift and the matrix shift are in reasonable agreement. The force field from the matrix dsta, however, agrees extremely well with the individual potentsal functions that Ginn et 21. [l] obtained with lo<44 and 11C44, the isotopii: zeta constants, as constraints; namely,
F34 = -0.325 and F34 = -0.384, respectively. TM agreement may be Fortuitous as a result of ~e-s~~ic~tly larger dispersion in the A”4
value. frequencies for the v4 modes are blue ‘shifted slightly ‘in *+e matrix; thal is, the gasphase fre?,uencles at 481.13 [l] (!%F9) and 479.36 cm-1 [I] (IlBF ) increase in the matrix to 482.7 md 480.8 cm*\, -respectively. Since ‘iviewould expect the matrix to perk-b the ~4 modes I ‘, : ‘I, _’ ,. _‘matrix
The
: ‘-id8
: .
:
_., ‘_,
,;,
-.
,.
”
of both isotopes equally, the slightly kirger value for Av4 in the matrix over that obtained in the gas-phase (1.9 to 1.77 cm-l) may again reflect the overlapping of the contours of the two iso-, tapes. In summary, Av4 for BF3 determined from the infrared matrix spectra agrees quite well with the gas-phase frequency shift obtained from high resoktion studies. This encour2ging result reinforces the general suggestion concerning the applicability of matrix information toward determining precise frequency shift parameters for use in constraining intramolecular force fields. REFERENCES 111 S. G. W. Giun, D. Johansen and J. Overend, J. Mol. Spectry. 36 (1970) 448. [Z] A. A. Chalmers and D. C.McKean, S&%3x~hidb. Acta 22 (1966) 251. [3] S. G. W. Ginn,. C. WI Brow& J.5. Kcinnegand J. Cverend, J.Mol. 3pcctry. 28 4968} SOS. ;’
‘-
., .‘. .‘-. :, ,.,,_ ’ : .‘, ,, .‘. ,’ (.” ‘1.. ., ; ,” : .’ _. ,’ . .I.: (.._. ., ,..‘( -. _, ..-, :_ :_ ,; .,. t,i ., ,,,,,./’ ,,’ .-,’ ‘..,I :_. (’ .. . :. ‘, :