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The He(I) photoelectron spectrum of benzyl alcohol-liquid surface and gas phase
Volume
126, number
CHEMICAL
3,4
PHYSICS
LETTERS
9 May 1986
THE He(I) PHOTOELECTRON SPECTRUM OF BENZYL ALCOHOL - LIQUID SURFACE AND GAS PHASE R.E. BALLARD, Jimmy JONES, Elizabeth SUTHERLAND, Derek READ and Andrew INCHLEY School of Chemical Sciences, Umversity of East Anglra, Norwich NR4 7TJ. Norfolk, Received
22 January
UK
1986
He(I) photoelectron spectra of benzyl alcohol (liquid surface and gas phase) are presented and a simple method is given for the removal of the effects of He(I& impurity. In addition to the general shift to lower ionisation energy usually observed in the spectrum of neutral (uncharged, as opposed to ionic) substances on change of phase from gas to liquid, there are also shifts in the ionisation energies of some bands relative to others: specifically the non-bonding 0 band shifts on liquefaction by 0.5 eV more than the other bands. This relative shift is attributed to hydrogen bonding.
1. Introduction In earlier work on the He(I) photoelectron spectrum of benzyl alcohol [l] it was not possible to detect in the liquid surface the second band in the spectrum of the gas molecule, assigned to the 0 ‘lone pair” orbital and this seeming absence of a band in the liquid phase spectrum was attributed to the involvement of the orbital in intermolecular H-bonding. However with improved instrumentation it appears quite clearly (B in fig. 1). In the photoelectron spectra of other involatlle alcohols (ethanediol, glycerol, etc.) pronounced band broadening and loss of structure is observed in the liquid and H-bonding has been implicated here also [2] . When an organic vapour condenses there is a decrease of about 1.5 eV in the threshold ionisation energy as a consequence of the electric polarisation of surrounding molecules in the condensed phase [3] ; the corresponding effect on the photoelectron spectrum is a general shift of all the peaks by the same amount and in the same direction. However should certain orbitals in the molecule interact with neighbouring molecules to a greater extent than others then they are expected to undergo additional shifts on this account, with the result that some bands in the spectrum of the condensed phase shift relative to the other bands. Published studies of the photoelectron spectra of dimers, e.g. water [4,5], methanol [6] and carboxylic acids [7, 0 009.2614/86/$0350 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
81, show quite clearly how the bands of the monomer split to produce a more complex dimer spectrum, but in a liquid the outcome is less predictable. It has long been known that relative shifting of bands in the UV spectra of organic compounds occurs on change of phase, e g. n + rr* transitions shift to the blue progressively on going from the gas phase to non-polar solvents like hexane and then to polar solvents like water, while a + IT*bands shift in the opposite direction [9,10].
2. Experimental. Photoelectron spectra The photoelectron spectrometer consists of a large, brass sample compartment into which the degassed liquid is introduced vertically from the top in the form of a jet emerging from a tube of stainless steel. It is possible to stop the flow so as to produce a static liquid surface - employing either a pendant dropletor alternatively the wetted surface of the tube. He(I) radiation can be directed horizontally so as to fall on the liquid surface and cause photoelectrons to enter a differentially pumped electron energy filter (a Bessel box). The potential on the Bessel box is controlled by a multichannel analyser (IX&G Ortec, type 7 100) which stores the spectra and passes them, via a BBC microcomputer, to the University’s mainframe computer in the form of a 512channel histogram. 311
Volume 126, number 3,4
CHEMICAL PHYSICS LETTERS
The gas phase results were measured by directing the ionising radiation so as just to miss a jet of the liquid and to pass through the vapour coming from the surface. By this method the gas phase spectra of involatile substances are measured at room temperature but good agreement was found with the earlier gas phase work in which a model PS16, Perkin-Elmer spectrometer with a heated probe was employed. Both sets of results were calibrated by the addition of standard gases (Ar, H20 and N,O). Since the helium lamp produces both He(I) radiation and a few percent of He(@) impurity, the spectra obtained as described above are contaminated with He(IP) “echoes” of the He(I) peaks which are removed by the following procedure. A line spectrum of a gas, for example Ar or N2 ,is measured under the same conditions as the spectrum to be corrected and the He(IP)/ He(I) ratio is determined from the relative heights of the lines and their echoes. The spectrum to be corrected is then multiplied by this ratio, the result is shifted along the energy axis by the appropriate energy difference (1.87 eV) and then subtracted, channel by channel, from the spectrum to be corrected.
9 May 1986
19
15 eV
3. Results and discussion Fig. 1 shows the spectra of benzyl alcohol (gas and liquid) before correction for He(IP), radiation; band B, formerly undetected, can be seen quite clearly as a consequence of improvements in instrumentation and it appears to be equivalent to band 2 of the gas spectrum. In order to dispel any suspicion that band B could be the He(IP) echo of the more intense, composite band labelled C, the correction procedure described above was applied; if it were merely an echo then B would not be present in the corrected spectrum but in fact it is still to be found there and substantially unaltered (fig. 2). The method of calibration of the energy scale of the liquid phase spectra has been described [2] and the error is thought to be of the same order as the resolution (+O.l ev). From the values in table 1 it can be seen that the shifts with respect to the gas for bands A and B are 1 .l and 1.5 eV respectively. Of greater accuracy than these absolute shifts (which inescapably suffer from doubts about the calibration procedure for the liquid-phase energy scale) is the relative shift of band B compared with band A; this amounts to 0.4 eV. 312
11
7
photoelectronspectrumof (lower)vapour and (upper)liquidsurfaceof benzyl alcohol. The spectra are offset
Fig. 1. He(I)
by 1.2 eV so as to bring A, C and D bands of the liquid into alignmentwith the correspondingbandsof the gas. Band B fails to come into alignmentsinceit has shifted 0.4 eV with respect to band A. The small peak at just below 7 eV is an echo of band A caused by He(@) impurity in the ionising radiation.
Much better again than the cbmparison of ionisation energies is the comparison of the entire spectra, as in fig. 2 for ionisation energies are derived from the peaks of spectra bands (single points only) and the bands of the liquid are broad enough in some cases to consist of 30 points or more. If the spectrum of the gas is placed beneath that of the liquid and shifted by 1.2 eV as in fig. 2, then band A overlaps its counterpart, band B fails to do so and the remaining bands C, D and E overlap their corresponding bands as well as band A does (as far as their composite nature will alIow a judgement). The general shift [3] in benzyl alcohol on liquefaction therefore amounts to 1.2 eV and there is a further shift of 0.4 eV for band B. On consideration that it is absent in toluene and close in energy to the corresponding band in methanol,
CHEMICAL PHYSICS LETTERS
Volume 126, number 3,4
9 May 1986
“lone pair” in energy by some 0.4 eV more than the bonding orbitals. It has long been believed that lone pairs are essential in all types of ~-bond~g [9] . ~~0~~ H-bond~g solvents cause the largest energy shifts of n + n* absorption bands (e.g. 0.25 eV for the acetone band [ lo,1 1] ) yet small effects of the same kind are found with any polar solvent. Lone pairs are particularly exposed to intermolecular effects of all types on account of their d~ection~ ch~acteristics - in the free molecule they lie along lines of minimum mass density where commonly the greatest increase takes place when the molecule finds itself in the closely packed en~ronment of a liquid or a solid phase - hence relative band shifts like that found here are to be expected. Without one or more physical properties which can be used to differentiate between orbitals described as “lone pair”, “ n-bonding”, etc., such designations are of little real value and it is to be hoped that the relative shift of a peak with respect to others in the spectrum on change of phase will provide a useful practical criterion. Fig. 2. Shop the result of correction for the presence of He(JP) impurity. Lower, the uncorrected spectrum of liquid benzyl alcohol and upper, the corrected spectrum. Although correction removes the He(&) echo the remainder of the speotrum is practically unchanged and it is concluded that the second band is genuinelypart of the He(J) spectrum rather than a He(Io) echo.
band B was assigned to the non-bonding oxygen orbital On [l] ; the composite A and C bands relate to bonding orbitals. From the above discussion it seems the effect of intermolecular interaction is to lower the Table 1 Verticaf ion&&on energies (ev) in the He(J) spectrum of benzyl alcohol. Shifts on condensation from gas to Squid are given for the fhst two peaks Peak
Gas
1 2 3 4 5 6 7 8 9
8.9 10.3 11.2 11.7 12.4 14.1 15.0 16.1 16.8
Liquid 7.8 (A) 8.8 (B) 10.7 (C) 12.5 (D) 13.3 (E)
shift 1.1 1.5
Ackmm4edgement We thank the Royal Society and the SEBC for generous grants, Mr. W. Plumbley and Mr. C. Ellis for the const~ction of apparatus.
References [ l] R.E. Ballard, G.G. Gunnel! and WP. Hagan, 3. Electron Spectry. 16 (1979) 435. [2] R.E. Ballard, J. Jones and E. Sutherland, Chem. Whys. Letters 112 (1984) 3%. 131 M. Batty, L.J. Johnston and LE. Lyons, Australian J. C&m. 23 (1970) 2397. (41 S. Tomoda and K. Kimura, Chem. whys. Letters 102 (1983) 560. [S] S. Tomoda, Y. Achiba and K. Kimura, Chem. whys. Letters 87 (1982) 197. [6] S, Tomoda and K. Kimura, Chem. Whys. 74 (1983) 121. [7J RX. Thomas, Proc. Roy. Sot. A331 (1972) 249. [8] S. Tomoda, Y. Achiba, N. Nomoto, K. Sato and K. Kimura, Chem. whys. 74 (1983) 113. [9] AP. WeBs, Structure inorganic chemistry (Chuendon press, Oxford, 1967) p. 287. [lo] J.N. MurreB, Theory of the electronic spectra of organic molecules (Methuen, London, 1963) p. 163. [ll] H. McConnell, J. Chem. whys. 20 (1952) 700.
313
126, number
CHEMICAL
3,4
PHYSICS
LETTERS
9 May 1986
THE He(I) PHOTOELECTRON SPECTRUM OF BENZYL ALCOHOL - LIQUID SURFACE AND GAS PHASE R.E. BALLARD, Jimmy JONES, Elizabeth SUTHERLAND, Derek READ and Andrew INCHLEY School of Chemical Sciences, Umversity of East Anglra, Norwich NR4 7TJ. Norfolk, Received
22 January
UK
1986
He(I) photoelectron spectra of benzyl alcohol (liquid surface and gas phase) are presented and a simple method is given for the removal of the effects of He(I& impurity. In addition to the general shift to lower ionisation energy usually observed in the spectrum of neutral (uncharged, as opposed to ionic) substances on change of phase from gas to liquid, there are also shifts in the ionisation energies of some bands relative to others: specifically the non-bonding 0 band shifts on liquefaction by 0.5 eV more than the other bands. This relative shift is attributed to hydrogen bonding.
1. Introduction In earlier work on the He(I) photoelectron spectrum of benzyl alcohol [l] it was not possible to detect in the liquid surface the second band in the spectrum of the gas molecule, assigned to the 0 ‘lone pair” orbital and this seeming absence of a band in the liquid phase spectrum was attributed to the involvement of the orbital in intermolecular H-bonding. However with improved instrumentation it appears quite clearly (B in fig. 1). In the photoelectron spectra of other involatlle alcohols (ethanediol, glycerol, etc.) pronounced band broadening and loss of structure is observed in the liquid and H-bonding has been implicated here also [2] . When an organic vapour condenses there is a decrease of about 1.5 eV in the threshold ionisation energy as a consequence of the electric polarisation of surrounding molecules in the condensed phase [3] ; the corresponding effect on the photoelectron spectrum is a general shift of all the peaks by the same amount and in the same direction. However should certain orbitals in the molecule interact with neighbouring molecules to a greater extent than others then they are expected to undergo additional shifts on this account, with the result that some bands in the spectrum of the condensed phase shift relative to the other bands. Published studies of the photoelectron spectra of dimers, e.g. water [4,5], methanol [6] and carboxylic acids [7, 0 009.2614/86/$0350 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
81, show quite clearly how the bands of the monomer split to produce a more complex dimer spectrum, but in a liquid the outcome is less predictable. It has long been known that relative shifting of bands in the UV spectra of organic compounds occurs on change of phase, e g. n + rr* transitions shift to the blue progressively on going from the gas phase to non-polar solvents like hexane and then to polar solvents like water, while a + IT*bands shift in the opposite direction [9,10].
2. Experimental. Photoelectron spectra The photoelectron spectrometer consists of a large, brass sample compartment into which the degassed liquid is introduced vertically from the top in the form of a jet emerging from a tube of stainless steel. It is possible to stop the flow so as to produce a static liquid surface - employing either a pendant dropletor alternatively the wetted surface of the tube. He(I) radiation can be directed horizontally so as to fall on the liquid surface and cause photoelectrons to enter a differentially pumped electron energy filter (a Bessel box). The potential on the Bessel box is controlled by a multichannel analyser (IX&G Ortec, type 7 100) which stores the spectra and passes them, via a BBC microcomputer, to the University’s mainframe computer in the form of a 512channel histogram. 311
Volume 126, number 3,4
CHEMICAL PHYSICS LETTERS
The gas phase results were measured by directing the ionising radiation so as just to miss a jet of the liquid and to pass through the vapour coming from the surface. By this method the gas phase spectra of involatile substances are measured at room temperature but good agreement was found with the earlier gas phase work in which a model PS16, Perkin-Elmer spectrometer with a heated probe was employed. Both sets of results were calibrated by the addition of standard gases (Ar, H20 and N,O). Since the helium lamp produces both He(I) radiation and a few percent of He(@) impurity, the spectra obtained as described above are contaminated with He(IP) “echoes” of the He(I) peaks which are removed by the following procedure. A line spectrum of a gas, for example Ar or N2 ,is measured under the same conditions as the spectrum to be corrected and the He(IP)/ He(I) ratio is determined from the relative heights of the lines and their echoes. The spectrum to be corrected is then multiplied by this ratio, the result is shifted along the energy axis by the appropriate energy difference (1.87 eV) and then subtracted, channel by channel, from the spectrum to be corrected.
9 May 1986
19
15 eV
3. Results and discussion Fig. 1 shows the spectra of benzyl alcohol (gas and liquid) before correction for He(IP), radiation; band B, formerly undetected, can be seen quite clearly as a consequence of improvements in instrumentation and it appears to be equivalent to band 2 of the gas spectrum. In order to dispel any suspicion that band B could be the He(IP) echo of the more intense, composite band labelled C, the correction procedure described above was applied; if it were merely an echo then B would not be present in the corrected spectrum but in fact it is still to be found there and substantially unaltered (fig. 2). The method of calibration of the energy scale of the liquid phase spectra has been described [2] and the error is thought to be of the same order as the resolution (+O.l ev). From the values in table 1 it can be seen that the shifts with respect to the gas for bands A and B are 1 .l and 1.5 eV respectively. Of greater accuracy than these absolute shifts (which inescapably suffer from doubts about the calibration procedure for the liquid-phase energy scale) is the relative shift of band B compared with band A; this amounts to 0.4 eV. 312
11
7
photoelectronspectrumof (lower)vapour and (upper)liquidsurfaceof benzyl alcohol. The spectra are offset
Fig. 1. He(I)
by 1.2 eV so as to bring A, C and D bands of the liquid into alignmentwith the correspondingbandsof the gas. Band B fails to come into alignmentsinceit has shifted 0.4 eV with respect to band A. The small peak at just below 7 eV is an echo of band A caused by He(@) impurity in the ionising radiation.
Much better again than the cbmparison of ionisation energies is the comparison of the entire spectra, as in fig. 2 for ionisation energies are derived from the peaks of spectra bands (single points only) and the bands of the liquid are broad enough in some cases to consist of 30 points or more. If the spectrum of the gas is placed beneath that of the liquid and shifted by 1.2 eV as in fig. 2, then band A overlaps its counterpart, band B fails to do so and the remaining bands C, D and E overlap their corresponding bands as well as band A does (as far as their composite nature will alIow a judgement). The general shift [3] in benzyl alcohol on liquefaction therefore amounts to 1.2 eV and there is a further shift of 0.4 eV for band B. On consideration that it is absent in toluene and close in energy to the corresponding band in methanol,
CHEMICAL PHYSICS LETTERS
Volume 126, number 3,4
9 May 1986
“lone pair” in energy by some 0.4 eV more than the bonding orbitals. It has long been believed that lone pairs are essential in all types of ~-bond~g [9] . ~~0~~ H-bond~g solvents cause the largest energy shifts of n + n* absorption bands (e.g. 0.25 eV for the acetone band [ lo,1 1] ) yet small effects of the same kind are found with any polar solvent. Lone pairs are particularly exposed to intermolecular effects of all types on account of their d~ection~ ch~acteristics - in the free molecule they lie along lines of minimum mass density where commonly the greatest increase takes place when the molecule finds itself in the closely packed en~ronment of a liquid or a solid phase - hence relative band shifts like that found here are to be expected. Without one or more physical properties which can be used to differentiate between orbitals described as “lone pair”, “ n-bonding”, etc., such designations are of little real value and it is to be hoped that the relative shift of a peak with respect to others in the spectrum on change of phase will provide a useful practical criterion. Fig. 2. Shop the result of correction for the presence of He(JP) impurity. Lower, the uncorrected spectrum of liquid benzyl alcohol and upper, the corrected spectrum. Although correction removes the He(&) echo the remainder of the speotrum is practically unchanged and it is concluded that the second band is genuinelypart of the He(J) spectrum rather than a He(Io) echo.
band B was assigned to the non-bonding oxygen orbital On [l] ; the composite A and C bands relate to bonding orbitals. From the above discussion it seems the effect of intermolecular interaction is to lower the Table 1 Verticaf ion&&on energies (ev) in the He(J) spectrum of benzyl alcohol. Shifts on condensation from gas to Squid are given for the fhst two peaks Peak
Gas
1 2 3 4 5 6 7 8 9
8.9 10.3 11.2 11.7 12.4 14.1 15.0 16.1 16.8
Liquid 7.8 (A) 8.8 (B) 10.7 (C) 12.5 (D) 13.3 (E)
shift 1.1 1.5
Ackmm4edgement We thank the Royal Society and the SEBC for generous grants, Mr. W. Plumbley and Mr. C. Ellis for the const~ction of apparatus.
References [ l] R.E. Ballard, G.G. Gunnel! and WP. Hagan, 3. Electron Spectry. 16 (1979) 435. [2] R.E. Ballard, J. Jones and E. Sutherland, Chem. Whys. Letters 112 (1984) 3%. 131 M. Batty, L.J. Johnston and LE. Lyons, Australian J. C&m. 23 (1970) 2397. (41 S. Tomoda and K. Kimura, Chem. whys. Letters 102 (1983) 560. [S] S. Tomoda, Y. Achiba and K. Kimura, Chem. whys. Letters 87 (1982) 197. [6] S, Tomoda and K. Kimura, Chem. Whys. 74 (1983) 121. [7J RX. Thomas, Proc. Roy. Sot. A331 (1972) 249. [8] S. Tomoda, Y. Achiba, N. Nomoto, K. Sato and K. Kimura, Chem. whys. 74 (1983) 113. [9] AP. WeBs, Structure inorganic chemistry (Chuendon press, Oxford, 1967) p. 287. [lo] J.N. MurreB, Theory of the electronic spectra of organic molecules (Methuen, London, 1963) p. 163. [ll] H. McConnell, J. Chem. whys. 20 (1952) 700.
313