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The Absorption Spectrum of a Supersonically Expanded Beam of Methanol in the Vacuum Ultraviolet Region
JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.
175, 234–238 (1996)
0028
The Absorption Spectrum of a Supersonically Expanded Beam of Methanol in the Vacuum Ultraviolet Region E. Sominska and A. Gedanken Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel, 2900 Received July 20, 1995
The vacuum ultraviolet absorption spectrum of methanol and two of its isotopes was recorded in the region 60 000– 70 000 cm01. The molecules were supersonically expanded and they absorbed the impinging photons 1–2 cm downstream from the pulsed nozzle. The sharp peaks indicate an efficient cooling process. However, we could not detect any new peaks associated with dimers or higher oligomers. We assign the observed peaks as transition to the 3p Rydberg states. q 1996 Academic Press, Inc.
INTRODUCTION
The absorption spectra of methanol recorded by Salahub and Sandorfy (1) (S&S) and Kaiser (2) are considered the only reliable spectra of methanol (3). In the region of interest, 60 000–70 000 cm01, two origins at 62 267 and 67 114 cm01 were observed. A rich vibrational structure was identified for both origins. These two origins were assigned by S&S (1) as the n0 r 3s and n0 r 3p transitions, respectively, where the n0 molecular orbital is predominately a nonbonding orbital centered on the oxygen atom (65%) and 3s and 3p are Rydberg molecular orbitals. Robin (3) preferred to assign them as the n0 r 3p and n0 r 3p* Rydberg transitions. The ionization potential of methanol is 88 420 cm01, which would yield term values of 26 153 and 21 306 cm01 for the two excited states. The first number is too large for a 3p term value and this casts some doubt on Robin’s interpretation. Electron impact and electron loss measurements were reported by several groups (4–7). Yoshidome and co-workers (6) have found and assigned transitions from the ground state to the triplet states which correspond to the abovementioned singlet Rydberg states. According to their interpretation, the splitting between the two triplets assigned to the 3p Rydbergs is 0.7 eV. This concurs with Robin’s assignment of the singlet 3p and 3p* Rydbergs, which also results in a splitting of 0.6 eV. This splitting of the two 3p components of the 3p manifold is exceptionally large and other examples of a similar magnitude are not known. In general the assignment proposed by all the electron impact groups agree with Robin’s interpretation. Wadt and Goddard (8) have performed improved virtual orbital calculations of the low-lying excited states of methanol. The first excited singlet state was the 3s Rydberg. The energies of the three components of the 3p manifold were calculated. Only two of the three 3p states carry oscillator
strength from the ground state. Those two states are separated by 0.43 eV. We have recently been involved in measuring the direct absorption of supersonically expanded molecules in the vacuum ultraviolet (VUV) region (9, 10). Our special interest was detecting absorptions of hydrogen-bonded dimers. We have succeeded in detecting such absorptions only for formic acid (10). We have failed in our attempts to detect the absorption of ammonia and acetic acid dimers and only the well-known absorption bands of the monomers were detected. The same results were also reported by another group (11). In the current paper we report the absorption spectra of CH3OH, CD3OD, and CH3OD. The rich vibrational structure is analyzed and assigned. The isotope effect is manifested in the spectra in energy shifts of the peaks as well as in their widths. EXPERIMENTAL
The apparatus which was employed in this investigation has been described elsewhere (9). In short, the light source is a modified Hintereger lamp through which hydrogen gas flows. To overcome the difficulty of the many sharp hydro˚ , an automatic gain control gen emission lines at l õ 1675 A feedback unit is employed. This unit adjusts the input voltage to the photomultiplier and keeps the dc signal constant at the desired level. The VUV radiation intersects a pulsed molecular beam 1–2 cm from the valve. The signal is fed into a lock-in amplifier or a box-car integrator. The reference signal to these instruments is taken from the pulsed valve ˚ signal. The accuracy of the peak positions at l õ 1675 A ˚ is 2 A. The supersonic expansion was carried out using the equilibrium vapor pressure of the compounds at room temperature. The CH3OH (Aldrich, 99% purity), the CH3OD (Ald-
234 0022-2852/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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AP: Mol Spec
ABSORPTION SPECTRUM OF METHANOL
235
˚. FIG. 1. The absorption spectrum of supersonically expanded CH3OH. The spectral resolution is 2.1 A
rich, 99.5% D atoms), and the CD3OD (Aldrich, 99.5% D atoms) were used without further purification. RESULTS AND DISCUSSION
(a) Vibrational Assignment The absorption spectra of methanol and its isotopes are presented in Figs. 1–3. All the spectra were recorded with the same slit width. Since we have observed changes in the widths of various bands upon isotopic substitution, we also ˚. measured the width of the origin band of CS2 at 1593 A This is one of the narrowest absorption bands in this energy range (12). The width of the CS2 band is taken as the instrumental resolution in this wavelength range. We adopt the proposed assignment of the spectrum as composed of at least two transitions to the 3p Rydberg states (3). The origins of these transitions in CH3OH according to ˚ ) and 67 114 (1490 A ˚) our results are: 62 286 (1605.5 A cm01. We will refer to these two transitions as the origins of the A and B states, respectively. The simpler case is that of the second origin, B, in which a short progression of 1020 cm01 for CH3OH, 1020 cm01 for CH3OD, and 960 cm01 for
CD3OD is detected. This vibration is assigned as the n8 (12), the CO stretching, whose ground state frequency is 1033 cm01. The assignment is based on the similarity between the frequency ratio of this vibration in the ground state of the three isotopes (CH3OH:CH3OD:CD3OD Å 1.0:1.0:1.05) and the measured ratio in the excited state (1.0:1.0:1.06). In all three isotopes we detect the origin plus two members of this progression. In CD3OD one additional band is observed at ˚ ), which cannot be associated with this 69 930 cm01 (1430 A progression because its energy spacing from the previous band is just 630 cm01. The vibrational structure of the lower energy component of the 3p manifold, state A, is more complicated. The first absorption peak, the origin, in each of the three isotopes is relatively narrow and unstructured. In CH3OH the second absorption peak shows a triplet structure, a central peak plus red and blue shoulders. The spacing between the origin ˚ ) and the central peak of the triplet structure at (1605.5 A ˚ 1584 A is 826 cm01. The second band in CH3OD is located ˚ and its spacing from the origin is 787 cm01. This at 1584 A band is broadened and we could not resolve any substructure even with slightly narrower slits. In CD3OD the second ab-
˚. FIG. 2. The absorption spectrum of supersonically expanded CH3OD. The spectral resolution is 2.1 A
Copyright q 1996 by Academic Press, Inc.
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SOMINSKA AND GEDANKEN
˚. FIG. 3. The absorption spectrum of supersonically expanded CD3OD. The spectral resolution is 2.1 A
˚ reveals a completely different structure. sorption at 1574 A It is also composed of a triplet which is well resolved and ˚ , is the most intense of the its first component, at 1580 A three bands. Its spacing from the origin is only 516 cm01. If we assign the 826 cm01 spacing in CH3OH as associated with the excitation of the CO stretching mode, then the 516 cm01 spacing in CD3OD cannot correspond to the same vibrational mode. This is because the frequency ratios of this vibration in the two isotopes n8(CH3OH):n8(CD3OD) in the ground state and in the B state are 1.05 while the ratio in the A excited state is 826/516 Å 1.6. This therefore leads ˚ in CD3OD with us to associate the weaker peak at 1572 A the CO stretching vibration. This band is separated from the origin by 838 cm01, a spacing that can still be associated with the CO stretching vibration (the ratio in the excited state would be 0.99 compared with 1.05 in the ground state). ˚ , whose spacing from On the other hand, the band at 1580 A 01 the origin is just 516 cm , will be assigned to the CD3 rocking vibration whose ground state frequency is 776 cm01 in CD3OD. We have preferred this vibration because this assignment requires the smallest change in the vibrational frequency as compared to its ground state frequency. Any other assignment would lead to a drastic drop in vibrational frequency upon the excitation of the molecule to the A state. Such a drastic reduction in the frequency of any vibration would mean considerable changes in the configuration of the molecule upon excitation to the A state. All the indications are that the opposite is true; namely, that only minor configurational changes occur upon the excitation of the oxygen nonbonding orbital to the 3p Rydberg states. This is also true for the ground state of the ion which is usually structurally related to the Rydberg states. The vibrational structure in the photoelectron spectrum is very similar to that of the A and B states except that the short progression in the CO stretching vibration is built on the true origin as well as on one quantum of 1270 cm01. This is the reason that we prefer assigning the CD3 rocking vibration as the
active vibration of the 0–0 / 516 cm01 absorption. Consistent with this suggestion, we find the corresponding band at ˚ in CH3OH is spaced 646 cm01 from the origin. This 1589 A vibration is also assigned to the CH3 rocking mode. The isotopic ratio of this vibration in the A state is 1.25 while in the ground state it is 1.36. In CH3OD the corresponding transition is not resolved and it lies under the broad 0–0 band. The second origin at 1270 cm01 in the photoelectron spectrum was assigned to the CH3 deformation which has a frequency of 1477 cm01 in the ground state. Although a second progression built of 800 cm01 intervals is not observed in the A or B states we cannot rule out the possibility ˚ band is the 0–0 / 1 CH3 deformation that the 1577 A vibration (n4). The progression of the CO stretching vibration is longest in CD3OD. This is due to the fact that the interaction with the underlying continuum in the A state region is the weakest for CD3OD. This is also reflected in the lineshape of the absorption bands, which are the sharpest in CD3OD. Since we can attribute all the observed bands to the monomeric form of methanol for the three isotopes, we have no evidence from the vibrational assignment for the absorption of the dimeric methanol. In Table 1 we present the energies and assignments of the observed bands in the three isotopes. (b) Isotope Shifts and the 3p Rydberg Manifold The origins of valence and Rydberg states are known to undergo blue shifts upon deuteration. This is also demonstrated in the 3p Rydbergs (A and B states) in methanol. In Table 2 we present the energies and spectral shifts of the origins of the 3p Rydberg states for the various isotopes of methanol. The results indicate that the blue shift is strongly dependent on the number of hydrogen atoms substituted by deuterium atoms. This can be rationalized when the energy of the origin is expressed by (ignoring unharmonicity)
Copyright q 1996 by Academic Press, Inc.
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ABSORPTION SPECTRUM OF METHANOL
TABLE 1 Energies and Assignment of the Absorption Bands in the Three Methanol Isotopes
TABLE 3 The Width of the Origin Band of the A State (cm01)
nol. The highest component of the 3p manifold is calculated to appear at 8.63 eV (8), while the proposed origin is observed at 8.76 eV. (c) Band Widths of the Origins of the A State
T00 Å te / 1/2{∑ n*i 0 ∑ n9i }, i
[1]
i
where te is the pure electronic term and n* and n9 are the excited and ground state vibrational frequencies. For each of the three isotopomers we have to introduce into Eq. [1] the values of the ground and excited state vibrations. The spectral shift for CH3OD will be T00(CH3OD) 0 T00(CH3OH) Å 1/2{∑ n*i (CH3OD) 0 i
∑ n*i (CH3OH)}
[2]
i
0 1/2{∑ n9i (CH3OD) 0 i
∑ n9i (CH3OH)}. i
The spectral shift contains only the frequency differences while the pure electronic term is invariant to isotopic substitution and is zeroed upon the subtraction. The two expressions in parentheses are negative and since the isotope energy shift is positive, the changes (upon deuteration) in the vibrational frequencies of the ground state have to be larger than those in the excited state. This is what usually happens in cases of deuterium substitution and is also found in our case. Table 2 includes a column of a third component of a 3p Rydberg, the C state. This origin is not related to the progression of the leading vibration and we offer its assignment as the missing component of the 3p manifold of metha-
TABLE 2 Isotopic Energy Shifts of the Origins of States A and B
Deuteration is known to decrease the rate of radiationless transitions. This is demonstrated either through the linewidth of absorption bands or through fluorescence quantum yield measurements. The narrowing of absorption transition upon deuteration was observed for Rydberg (15) as well as valence transitions (16). This behavior is observed in the isotopes of methanol as well. In Table 3 we present the widths of the origin of the A state for all three isotopes. CONCLUSION
Methanol vapors at room temperature are known to be composed of monomers, dimers, and tetramers (17, 18). We could not observe new peaks in our absorption spectrum that were not detected previously in the monomer spectrum (17). One possible reason for these observations is perhaps due to the shapes of the potential energy curves of the ground and excited states of the dimers. If these two potential curves are similar, namely, no major changes in the dimer’s configuration occur upon excitation, then only the 0–0 transition should be observed. The dimer 0–0 itself might underlie the broad origin of the monomer. We have, for the first time, presented and assigned the absorption spectra of two deuterated methanol molecules. ACKNOWLEDGMENTS We thank the Israel Academy of Science and Humanities for supporting this research through a grant administered by the Israel Science Foundation. Dr. E. Sominska also thanks the Ministry of Science and Technology, the Ministry of Absorption, and the Center for Absorption in Science.
REFERENCES 1. D. R. Salahub and C. Sandorfy, Chem. Phys. Lett. 8, 71–74 (1971). 2. H. Kaiser, Ph.D. Thesis, Ludwig-Maximilian University, Munchen, Germany, 1970. 3. M. B. Robin, ‘‘Higher Excited States of Polyatomic Molecules,’’ Vol. I, p. 254. Academic Press, New York, 1974. 4. W. C. Tam and C. E. Brion, J. Electron. Spectrosc. Relat. Phenom. 3, 263–279 (1974). 5. T. Ogawa, T. Yoshidome, and H. Kawazumi, Chem. Lett., 115–118 (1990).
Copyright q 1996 by Academic Press, Inc.
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SOMINSKA AND GEDANKEN
6. T. Yoshidome, H. Kawazumi, and T. Ogawa, J. Electron. Spectrosc. Relat. Phenom. 53, 185–192 (1990). 7. M. B. Robin and N. A. Kuebler, J. Electron. Spectrosc. Relat. Phenom. 1, 13–28 (1972). 8. W. R. Wadt and W. A. Goddard, Chem. Phys. 18, 1–11 (1976). 9. R. Mualem and A. Gedanken, Chem. Phys. Lett. 188, 383–336 (1992). 10. R. Mualem, E. Sominska, V. Kelner, and A. Gedanken, J. Chem. Phys. 98, 8813–8814 (1992). 11. J. A. Syage, R. B. Cohen, and J. Steadman, J. Chem. Phys. 97, 6072– 6084 (1992). 12. R. McDiarmid and J. P. Doering, J. Chem. Phys. 91, 2010–2085 (1989).
13. E. Sominska, V. Kelner, and A. Gedanken, J. Phys. Chem. 96, 10,240– 10,242 (1992). 14. The numbering of the vibrations is according to T. Shimanuchi, ‘‘Tables of Vibrational Frequencies.’’ NSRDS, Washington, DC, 1972. 15. R. McDiarmid and A. Sabljic, J. Phys. Chem. 91, 276–282 (1987). 16. R. McDiarmid and A-H. Sheybani, J. Chem. Phys. 89, 1255–1261 (1988). 17. R. G. Inskeep, J. M. Kelliher, P. E. McMahon, and B. G. Somers, J. Chem. Phys. 28, 1033–1036 (1958). 18. C. B. Kretscmer and R. Wiede, J. Am. Chem. Soc. 76, 2579–2083 (1954).
Copyright q 1996 by Academic Press, Inc.
AID
JMS 6913
/
m4652$$$88
01-18-96 00:57:51
mspa
AP: Mol Spec
175, 234–238 (1996)
0028
The Absorption Spectrum of a Supersonically Expanded Beam of Methanol in the Vacuum Ultraviolet Region E. Sominska and A. Gedanken Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel, 2900 Received July 20, 1995
The vacuum ultraviolet absorption spectrum of methanol and two of its isotopes was recorded in the region 60 000– 70 000 cm01. The molecules were supersonically expanded and they absorbed the impinging photons 1–2 cm downstream from the pulsed nozzle. The sharp peaks indicate an efficient cooling process. However, we could not detect any new peaks associated with dimers or higher oligomers. We assign the observed peaks as transition to the 3p Rydberg states. q 1996 Academic Press, Inc.
INTRODUCTION
The absorption spectra of methanol recorded by Salahub and Sandorfy (1) (S&S) and Kaiser (2) are considered the only reliable spectra of methanol (3). In the region of interest, 60 000–70 000 cm01, two origins at 62 267 and 67 114 cm01 were observed. A rich vibrational structure was identified for both origins. These two origins were assigned by S&S (1) as the n0 r 3s and n0 r 3p transitions, respectively, where the n0 molecular orbital is predominately a nonbonding orbital centered on the oxygen atom (65%) and 3s and 3p are Rydberg molecular orbitals. Robin (3) preferred to assign them as the n0 r 3p and n0 r 3p* Rydberg transitions. The ionization potential of methanol is 88 420 cm01, which would yield term values of 26 153 and 21 306 cm01 for the two excited states. The first number is too large for a 3p term value and this casts some doubt on Robin’s interpretation. Electron impact and electron loss measurements were reported by several groups (4–7). Yoshidome and co-workers (6) have found and assigned transitions from the ground state to the triplet states which correspond to the abovementioned singlet Rydberg states. According to their interpretation, the splitting between the two triplets assigned to the 3p Rydbergs is 0.7 eV. This concurs with Robin’s assignment of the singlet 3p and 3p* Rydbergs, which also results in a splitting of 0.6 eV. This splitting of the two 3p components of the 3p manifold is exceptionally large and other examples of a similar magnitude are not known. In general the assignment proposed by all the electron impact groups agree with Robin’s interpretation. Wadt and Goddard (8) have performed improved virtual orbital calculations of the low-lying excited states of methanol. The first excited singlet state was the 3s Rydberg. The energies of the three components of the 3p manifold were calculated. Only two of the three 3p states carry oscillator
strength from the ground state. Those two states are separated by 0.43 eV. We have recently been involved in measuring the direct absorption of supersonically expanded molecules in the vacuum ultraviolet (VUV) region (9, 10). Our special interest was detecting absorptions of hydrogen-bonded dimers. We have succeeded in detecting such absorptions only for formic acid (10). We have failed in our attempts to detect the absorption of ammonia and acetic acid dimers and only the well-known absorption bands of the monomers were detected. The same results were also reported by another group (11). In the current paper we report the absorption spectra of CH3OH, CD3OD, and CH3OD. The rich vibrational structure is analyzed and assigned. The isotope effect is manifested in the spectra in energy shifts of the peaks as well as in their widths. EXPERIMENTAL
The apparatus which was employed in this investigation has been described elsewhere (9). In short, the light source is a modified Hintereger lamp through which hydrogen gas flows. To overcome the difficulty of the many sharp hydro˚ , an automatic gain control gen emission lines at l õ 1675 A feedback unit is employed. This unit adjusts the input voltage to the photomultiplier and keeps the dc signal constant at the desired level. The VUV radiation intersects a pulsed molecular beam 1–2 cm from the valve. The signal is fed into a lock-in amplifier or a box-car integrator. The reference signal to these instruments is taken from the pulsed valve ˚ signal. The accuracy of the peak positions at l õ 1675 A ˚ is 2 A. The supersonic expansion was carried out using the equilibrium vapor pressure of the compounds at room temperature. The CH3OH (Aldrich, 99% purity), the CH3OD (Ald-
234 0022-2852/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AID
JMS 6913
/
m4652$$$87
01-18-96 00:57:51
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AP: Mol Spec
ABSORPTION SPECTRUM OF METHANOL
235
˚. FIG. 1. The absorption spectrum of supersonically expanded CH3OH. The spectral resolution is 2.1 A
rich, 99.5% D atoms), and the CD3OD (Aldrich, 99.5% D atoms) were used without further purification. RESULTS AND DISCUSSION
(a) Vibrational Assignment The absorption spectra of methanol and its isotopes are presented in Figs. 1–3. All the spectra were recorded with the same slit width. Since we have observed changes in the widths of various bands upon isotopic substitution, we also ˚. measured the width of the origin band of CS2 at 1593 A This is one of the narrowest absorption bands in this energy range (12). The width of the CS2 band is taken as the instrumental resolution in this wavelength range. We adopt the proposed assignment of the spectrum as composed of at least two transitions to the 3p Rydberg states (3). The origins of these transitions in CH3OH according to ˚ ) and 67 114 (1490 A ˚) our results are: 62 286 (1605.5 A cm01. We will refer to these two transitions as the origins of the A and B states, respectively. The simpler case is that of the second origin, B, in which a short progression of 1020 cm01 for CH3OH, 1020 cm01 for CH3OD, and 960 cm01 for
CD3OD is detected. This vibration is assigned as the n8 (12), the CO stretching, whose ground state frequency is 1033 cm01. The assignment is based on the similarity between the frequency ratio of this vibration in the ground state of the three isotopes (CH3OH:CH3OD:CD3OD Å 1.0:1.0:1.05) and the measured ratio in the excited state (1.0:1.0:1.06). In all three isotopes we detect the origin plus two members of this progression. In CD3OD one additional band is observed at ˚ ), which cannot be associated with this 69 930 cm01 (1430 A progression because its energy spacing from the previous band is just 630 cm01. The vibrational structure of the lower energy component of the 3p manifold, state A, is more complicated. The first absorption peak, the origin, in each of the three isotopes is relatively narrow and unstructured. In CH3OH the second absorption peak shows a triplet structure, a central peak plus red and blue shoulders. The spacing between the origin ˚ ) and the central peak of the triplet structure at (1605.5 A ˚ 1584 A is 826 cm01. The second band in CH3OD is located ˚ and its spacing from the origin is 787 cm01. This at 1584 A band is broadened and we could not resolve any substructure even with slightly narrower slits. In CD3OD the second ab-
˚. FIG. 2. The absorption spectrum of supersonically expanded CH3OD. The spectral resolution is 2.1 A
Copyright q 1996 by Academic Press, Inc.
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SOMINSKA AND GEDANKEN
˚. FIG. 3. The absorption spectrum of supersonically expanded CD3OD. The spectral resolution is 2.1 A
˚ reveals a completely different structure. sorption at 1574 A It is also composed of a triplet which is well resolved and ˚ , is the most intense of the its first component, at 1580 A three bands. Its spacing from the origin is only 516 cm01. If we assign the 826 cm01 spacing in CH3OH as associated with the excitation of the CO stretching mode, then the 516 cm01 spacing in CD3OD cannot correspond to the same vibrational mode. This is because the frequency ratios of this vibration in the two isotopes n8(CH3OH):n8(CD3OD) in the ground state and in the B state are 1.05 while the ratio in the A excited state is 826/516 Å 1.6. This therefore leads ˚ in CD3OD with us to associate the weaker peak at 1572 A the CO stretching vibration. This band is separated from the origin by 838 cm01, a spacing that can still be associated with the CO stretching vibration (the ratio in the excited state would be 0.99 compared with 1.05 in the ground state). ˚ , whose spacing from On the other hand, the band at 1580 A 01 the origin is just 516 cm , will be assigned to the CD3 rocking vibration whose ground state frequency is 776 cm01 in CD3OD. We have preferred this vibration because this assignment requires the smallest change in the vibrational frequency as compared to its ground state frequency. Any other assignment would lead to a drastic drop in vibrational frequency upon the excitation of the molecule to the A state. Such a drastic reduction in the frequency of any vibration would mean considerable changes in the configuration of the molecule upon excitation to the A state. All the indications are that the opposite is true; namely, that only minor configurational changes occur upon the excitation of the oxygen nonbonding orbital to the 3p Rydberg states. This is also true for the ground state of the ion which is usually structurally related to the Rydberg states. The vibrational structure in the photoelectron spectrum is very similar to that of the A and B states except that the short progression in the CO stretching vibration is built on the true origin as well as on one quantum of 1270 cm01. This is the reason that we prefer assigning the CD3 rocking vibration as the
active vibration of the 0–0 / 516 cm01 absorption. Consistent with this suggestion, we find the corresponding band at ˚ in CH3OH is spaced 646 cm01 from the origin. This 1589 A vibration is also assigned to the CH3 rocking mode. The isotopic ratio of this vibration in the A state is 1.25 while in the ground state it is 1.36. In CH3OD the corresponding transition is not resolved and it lies under the broad 0–0 band. The second origin at 1270 cm01 in the photoelectron spectrum was assigned to the CH3 deformation which has a frequency of 1477 cm01 in the ground state. Although a second progression built of 800 cm01 intervals is not observed in the A or B states we cannot rule out the possibility ˚ band is the 0–0 / 1 CH3 deformation that the 1577 A vibration (n4). The progression of the CO stretching vibration is longest in CD3OD. This is due to the fact that the interaction with the underlying continuum in the A state region is the weakest for CD3OD. This is also reflected in the lineshape of the absorption bands, which are the sharpest in CD3OD. Since we can attribute all the observed bands to the monomeric form of methanol for the three isotopes, we have no evidence from the vibrational assignment for the absorption of the dimeric methanol. In Table 1 we present the energies and assignments of the observed bands in the three isotopes. (b) Isotope Shifts and the 3p Rydberg Manifold The origins of valence and Rydberg states are known to undergo blue shifts upon deuteration. This is also demonstrated in the 3p Rydbergs (A and B states) in methanol. In Table 2 we present the energies and spectral shifts of the origins of the 3p Rydberg states for the various isotopes of methanol. The results indicate that the blue shift is strongly dependent on the number of hydrogen atoms substituted by deuterium atoms. This can be rationalized when the energy of the origin is expressed by (ignoring unharmonicity)
Copyright q 1996 by Academic Press, Inc.
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JMS 6913
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m4652$$$88
01-18-96 00:57:51
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AP: Mol Spec
237
ABSORPTION SPECTRUM OF METHANOL
TABLE 1 Energies and Assignment of the Absorption Bands in the Three Methanol Isotopes
TABLE 3 The Width of the Origin Band of the A State (cm01)
nol. The highest component of the 3p manifold is calculated to appear at 8.63 eV (8), while the proposed origin is observed at 8.76 eV. (c) Band Widths of the Origins of the A State
T00 Å te / 1/2{∑ n*i 0 ∑ n9i }, i
[1]
i
where te is the pure electronic term and n* and n9 are the excited and ground state vibrational frequencies. For each of the three isotopomers we have to introduce into Eq. [1] the values of the ground and excited state vibrations. The spectral shift for CH3OD will be T00(CH3OD) 0 T00(CH3OH) Å 1/2{∑ n*i (CH3OD) 0 i
∑ n*i (CH3OH)}
[2]
i
0 1/2{∑ n9i (CH3OD) 0 i
∑ n9i (CH3OH)}. i
The spectral shift contains only the frequency differences while the pure electronic term is invariant to isotopic substitution and is zeroed upon the subtraction. The two expressions in parentheses are negative and since the isotope energy shift is positive, the changes (upon deuteration) in the vibrational frequencies of the ground state have to be larger than those in the excited state. This is what usually happens in cases of deuterium substitution and is also found in our case. Table 2 includes a column of a third component of a 3p Rydberg, the C state. This origin is not related to the progression of the leading vibration and we offer its assignment as the missing component of the 3p manifold of metha-
TABLE 2 Isotopic Energy Shifts of the Origins of States A and B
Deuteration is known to decrease the rate of radiationless transitions. This is demonstrated either through the linewidth of absorption bands or through fluorescence quantum yield measurements. The narrowing of absorption transition upon deuteration was observed for Rydberg (15) as well as valence transitions (16). This behavior is observed in the isotopes of methanol as well. In Table 3 we present the widths of the origin of the A state for all three isotopes. CONCLUSION
Methanol vapors at room temperature are known to be composed of monomers, dimers, and tetramers (17, 18). We could not observe new peaks in our absorption spectrum that were not detected previously in the monomer spectrum (17). One possible reason for these observations is perhaps due to the shapes of the potential energy curves of the ground and excited states of the dimers. If these two potential curves are similar, namely, no major changes in the dimer’s configuration occur upon excitation, then only the 0–0 transition should be observed. The dimer 0–0 itself might underlie the broad origin of the monomer. We have, for the first time, presented and assigned the absorption spectra of two deuterated methanol molecules. ACKNOWLEDGMENTS We thank the Israel Academy of Science and Humanities for supporting this research through a grant administered by the Israel Science Foundation. Dr. E. Sominska also thanks the Ministry of Science and Technology, the Ministry of Absorption, and the Center for Absorption in Science.
REFERENCES 1. D. R. Salahub and C. Sandorfy, Chem. Phys. Lett. 8, 71–74 (1971). 2. H. Kaiser, Ph.D. Thesis, Ludwig-Maximilian University, Munchen, Germany, 1970. 3. M. B. Robin, ‘‘Higher Excited States of Polyatomic Molecules,’’ Vol. I, p. 254. Academic Press, New York, 1974. 4. W. C. Tam and C. E. Brion, J. Electron. Spectrosc. Relat. Phenom. 3, 263–279 (1974). 5. T. Ogawa, T. Yoshidome, and H. Kawazumi, Chem. Lett., 115–118 (1990).
Copyright q 1996 by Academic Press, Inc.
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Copyright q 1996 by Academic Press, Inc.
AID
JMS 6913
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m4652$$$88
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AP: Mol Spec