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Evidence from inelastic electron tunnelling spectroscopy for vibrational mode reassignments in simple aliphatic carboxylate ions
Spectrochimica Acfa, Vol. 37A. No. Printed in Great Britain.
11, pp. 1015-1019.
058447539/81/111015-05$02.00/0 01981 Pergamon PressLtd.
1981
Evidence from inelastic electron tunnelling spectroscopy for vibrational mode reassignments in simple aliphatic carboxylate ions D. G. WALMSLEY, W. J. NELSON and N. M. D. BROWN School of Physical Sciences, New University of Ulster, Coleraine BT52 ISA, U.K. and
S. DE CHEVEIGN~, S. GAUTHIER, J. KLEIN and A. LACER Groupe de Physique des Solides de I’Ecble Normale Superieure, Tour 23-2, place Jussieu-75221, Paris Cedex 05, France (Receioed
18 April 1981)
Abstract-Tunnelling studies of the vibrational spectra of formate- and acetate-, including ‘soenriched acetate-, anions adsorbed on plasma-grown aluminium and magnesium oxide surfaces show that the COz- symmetric stretch mode can be clearly distinguished from the CH modes in the same region of the spectra. This CO*- mode of the formate ion is at 1456 cm-’ on aluminium oxide and at 1348 cm-’ on magnesium oxide: for the acetate ion the corresponding values are 1463 and 1445 cm-‘. Very well resolved vibrational spectra of surfaceadsorbed species are seen in inelastic electron tunnelling spectroscopy[ I-41. As part of a wider series of investigations using the technique we have examined the spectra of some simple aliphatic carboxylate ions adsorbed on metal oxides. The interpretation of these spectra has led us to reassess the usual assignments for the carboxylate moiety symmetric stretch vibrations and make firm suggestions in the cases of the formate and acetate anions. First consider the spectrum obtained when formic acid is adsorbed on plasma-grown aluminium oxide (Fig. la). Background structure at 940 cm-’ is due to aluminium oxide and at -36OOcm-’ is due to hydroxyl species present in the oxide. Both features have been extensively discussed in the literature of tunnelling spectroscopy and need not concern us here. Reaction of the acid on the surface to give the formate anion is suggested by the absence of an identifiable acid OH stretch mode and confirmed by the absence of the C=O stretch mode at 1780cm-’ and the C-OH stretch mode at -1100 cm-‘[5]. Instead we see clearly the broad CO*- antisymmetric stretch mode of the formate anion at 1580 cm-’ (Table 1). The CH stretch mode is found at 2875 cm-‘, the CH out-of-plane bend mode is strong at 1038 cm-’ and the CO,- rock mode is probably that seen at 242cm-‘. There remains only the question of which of the modes at 1370 and 1456 cm-’ is the CH in-plane bend and which is the CO,- symmetric stretch. The problem is resolved as follows. By analogy with i.r. and IETS assignments on higher carboxylates we initially assign the 1456cm-’ band as the CO,- symmetric stretch mode. When we examine the tunnel spectrum of the formate ion on a different surface, magnesium oxide, we find (Fig. lb) that the band at 1456cm-’
Table 1. IETS vibrational modes of (a) HCOO- on plasma-grown aluminium oxide, (b) HCOO- on plasmagrown magnesium oxide and (c) DCOO- on plasma-grown aluminium oxide
(4
@I
(cl
242
236
256 887
1038
1046
1016
1348 1370 1456 1580
1575
2724 2875 2991
2709 2815 2976
1331 1452 1589 2162 2885 2950
Assignment CO*- rock CD out-of-plane bend CH out-of-plane bend I (+ CD in-plane bend?) (CO,- symm. stretch \ +Cfi in-plane bend CH in-mane bend COz- &mm. stretch CO*- antisymm. stretch CD stretch CH in-plane bend overtone CH stretch
has disappeared but the 1370cm-’ band has grown in strength and is slightly shifted down in energy to 1348 cm-‘. We do not expect to find substantial shifts in the internal modes of the adsorbed anion simply as a consequence of a different local molecular environment; only van der Waals forces are involved and these alone do not cause noticeable shifts, for example, in bulk liquid samples (although, of course, electrostatic and hydrogen bonding interactions do). However, at the actual site of chemisorption the dominant interaction occurs between the adsorbed anion and a cation of the local oxide surface. We may anticipate a frequency shift in the modes of the carboxylate moiety of the anion since the carbon-oxygen bond characteristics will alter with different oxygenmetal cation interactions on the two different oxide surfaces. The CO,- symmetric stretch mode has an associated oscillating dipole moment per1015
D. G.
1016
WALMSLEY et al.
N D
3
N
T)
0
800
1600
LLUU c
3‘“”
me’
Fig. 1. Tunnel spectra of (a) HCOO- on aluminium oxide, (b) HCOO- on magnesium oxide and (c) DCOO- on aluminium oxide.
pendicular to the surface and is therefore likely to be more sensitive to the adsorbate-adsorbent interaction than the CO,- antisymmetric stretch mode with its nett dipole oscillating parallel to the surface. It is therefore plausible to conclude that the CO,- symmetric mode has shifted down to coincide with the original undisturbed CH mode at - 1370 cm-’ or to fall slightly below it when the acid is adsorbed on magnesium oxide. The shift is in the correct sense when the more ionic nature of magnesium oxide, as compared with aluminium oxide, is recognized. (A barely discernable downward shift amounting to -5 cm-’ is found for the CO*- antisymmetric mode.) Complementary support for the identification comes from the spectrum of deuterated (‘H2)
formic acid adsorbed on aluminium oxide (Fig. lc). Here the band at 1370 cm-’ which we attributed to the CH in-plane bend mode in the undeuterated species is much reduced in intensity. The persistence of this band is a consequence of the residual undeuterated formic acid present in the deuterated sample used (nominally -98%). The apparently high relative intensity in this case arises from the intrinsically greater interaction matrix elements expected for C-H modes compared with C-2H. It is not expected to disappear completely because some exchange is inevitable between the deuterium and surface protons; a residual CH stretch band bears witness to this process. In addition, both supposed carboxylate modes are higher in energy by approximately the same
Inelastic electron tunnelling spectroscopy amount (-20cm-‘) when compared with the undeuterated species. MAAS[6] recently summarized previous i.r. work on sodium formate and notes the challenge of assigning the CH in-plane bend and CO,- symmetric stretch modes. Studies of monocrystals with polarized radiation[7,8] led to the lower energy mode being attributed to the CH bend and the upper to the CO,- symmetric stretch as suggested by the tunnelling results; opposite attributions were made by other authors[9-121 using more standard techniques with polycrystalline samples. These and other i.r. studies of the formate ion[l3,14] show a narrow band near 1370 cm-’ but there is always a very broad band in the 145&1700 cm-’ region which clearly embraces the CO,- antisymmetric mode and may also include the band we have identified as CO,- symmetric stretch. The breadth of this band is not set by sampling considerations or instrument resolution which, for these samples, can be as good as 3, or even 1 cm-‘, but is a real linewidth. The broad band observed clearly results from a combination of thermal and local environmental contributions such as hydrogen-bonding. The tunnel spectra are taken with the samples at low temperature (2 K) to obtain sufficient instrument resolution and this low sample temperature may explain why the bands are better resolved than in i.r.; i.r. measurements on formates at low temperatures would be of interest. JONES and McLAREN[IS] found better resolution of acetate i.r. spectra at 80 K. The tunnel spectra of the propanoate and benzoate ions adsorbed on aluminium oxide and magnesium oxide have been compared in an earlier paper[l6]. They too show a substantial downward shift in energy of one band which we identify as the CO,- symmetric stretch mode. The shift is much smaller in both than observed in the formate. For propanoate ion the band moves from 1444cm-’ on aluminium oxide to 1417 cm-’ on magnesium oxide while for benzoate ion the corresponding positions are 1428 and 1396 cm-‘. A more extensive check by tunnelling has now been made on the acetate ion. Its spectrum when adsorbed on aluminium oxide may be seen in Fig. 2a. The COZ- antisymmetric stretch mode is seen clearly at 1583 cm-‘. Following the procedure of the formate case we attribute the band at 1463 cm-’ to the COZ- symmetric mode with a CH, deformation at 1419cm-‘. On a magnesium oxide surface this supposed CO,- symmetric mode has moved down in energy to 1445 cm-‘. The position of the peak of the CO*- antisymmetric mode has also apparently moved down from 1583 to 1565 cm-’ but this is a broad band and the position and width of the base of the band are the same (to within 5 cm-‘) in both spectra and we therefore regard it as not having shifted to a significant extent. Interestingly too the band at 689cm-’ on aluminium oxide which we earlier identified [ 171 as
1017
CO,- symmetric deformation also shows a downward movement to 662 cm-’ on magnesium oxide. All other bands are at substantially the same energy on both surfaces (Table 2). Further confidence in the assignment of the 1463 and 1419cm-’ bands comes from the spectra of CD,COO- and CF,COO- adsorbates on aluminium oxide[l7]. The 1419cm-’ is entirely absent from both as would be expected if it is due to a CH, mode. Final confirmation of the identification of the CO,- symmetric stretch mode comes in convincing fashion from a study of the spectrum of “Oenriched acetate ion on aluminium oxide. The spectrum is shown as Fig. 2c and peak positions are recorded in Table 2. The most notable change on going from I60 to I80 acetate involves the 1463 cm-’ peak, which is shifted down into the 1419 cm-’ peak: the result is a peak at 1403 cm-’ (-4.3%). The sensitivity of the 1463 cm-’ mode to change in the oxygen mass clearly identifies it as the C02stretching mode. The 1419 cm-’ peak, insensitive to the mass change, is therefore attributed to the CH3 deformation mode. Consistently, the other modes involving oxygen atoms are also shifted: the CO,- antisymmetric stretch from 1597 to 1573 cm-’ (-1.5%) and the CO,- scissor mode from 678 to 653 cm-’ (-3.8%). These relative shifts[l8] are somewhat smaller than those observed for the symmetric stretch, as expected for a nonlinear triatomic unit [19]. On the other hand, the modes involving the CH, group are essentially unaffected, although the separation between the two CH, rocking modes near 1040cm-’ increases slightly. The CH, deformation mode at 1419 cm-’ is hidden by the new 1403 cm-’ peak. The CO,- rocking modes at 468 and 615 cm-’ involve movement of the CH, moiety with respect to the CO2 moiety and are unaffected. Table 2. IETS vibrational modes of (a) CH$OOon plasma-grown aluminium oxide (b) CH$ZOO- on plasmagrown magnesium oxide and (c) ‘*O-enriched CH,COO- on plasma-grown aluminium oxide (a)
(b)
407 426 468 471 618 615 6891678 662 945 938 1024 1023 1051 1046 1341 1343 1372(sh) 1419 1417 1463 1445 1583/1597 1565 2866 2865 2914 2926 2964 2977 3lnM 3007 -3640 -3650
(c) 403 468 605 653 919 1016 1048 1339 1403 1573 2919 2968 3000 I
Assignment
-
COz- in-plane rock CO*- out-of-plane rock CO,- symm. def. C-C stretch CH2 rock CHg rock CH, def. CH3 def. CO*- symm. stretch CO?,- antisymm. stretch CH stretch OH stretch
1018
D. G. WALMSLEY et al.
0
800
1600
c m-’
2LOO
3200
Fig. 2. Tunnel spectra of (a) CH,COO- on aluminium oxide, (b) CH#ZOO- on magnesium oxide and (c) ‘*O-enriched CH#ZOO- on aluminium oxide.
The analysis of the effects of changing the mass of the oxygen atoms in acetic acid clearly shows that the 1463 cm-’ mode involves the movement of oxygen. We therefore assign it to the symmetric stretch of COz-, and the mode at 1419cm-’ to the CH, deformation. In careful i.r. studies of sodium acetate in which the samples were cooled to 80 K to improve resolution JONES and McLAREN[IS] assigned the COzsymmetric stretch as 1408 cm-‘. Shortly after, WILMHURST[~~] argued, by analogy with related compounds, that a better assignment would be the CO,- symmetric stretch at 1422cm-‘. More recently ALCOCK, TRACY and WADDINGTON[21] found the CO,symmetric stretch in
Al(CH,COO-), at 1465 cm-’ which is in excellent agreement with the tunnelling data. In view of the consistent behaviour seen in all the tunnel spectra with a variety of carboxylates chemisorbed on two different oxides there is little room for ambiguity in the interpretation. A satisfying feature of the assignments we propose here is the relative constancy of the CO,- symmetric stretch mode in all the aluminium carboxylates, down to the lowest member of the series. Our conclusions are strongly supported by the earlier tunnelling work of SHKLYAREVSKIIef al. [22] who studied formic and acetic acids, both physisorbed and chemisorbed, on aluminium and magnesium oxides.
Inelastic electron tunnelling spectroscopy Acknowledgements-This work has been supported by CNRS, SRC, Northern Ireland Department of Education and Imperial Chemical Industries Ltd.
REFERENCFS P. K. HANSMA, Phys. Rep. 3OC, 145 (1977). 121 _ W. H. WEINBERG, Ann. Rev. Phvs. Chem. 29. 115 (1978). [3] P. K. HANSMA and J. KIRTLEY, Accts. Chem. Res. [l]
[14] Y.
NOTO,
K.
Trans Faraday
1019 FUKUDA, T. ONISHI and K. TAMARU,
Sot. 63, 2300 (1967).
[15] L. H. JONES and E. MCLAREN, J. Chem. Phys. 22, 1796 (1954). [16] D. G. WALMSLEY, W. J. NELSON, N. M. D. BROWN and R. B. FLOYD. Aool. Surf. Sci. 5. 107 (1980). [17] N. M. D. BROWN,’ R. i. FLOYD and d. G. WALMSLEY,J. Chem. Sot., Faraday Trans II 75, 17 (1979). [18] The CO*- antisymmetric
170 (1956). [lo] K. B. HARVEY,
stretch mode gives a broad band with its peak position rather sensitively dependent on the details of sample preparation, in particular the precise conditions of oxide growth. The two values (1583/1597 cm-‘) listed in Table 2 for this mode represent distinct reproducible results obtained in our two laboratories. The shifts observed also are reproducible. Like comments apply to the CO*- symmetric deformation mode at 689/678 cm-’ but the CO,- svmmetric stretch and all other modes of the adsorbed anion are unambiguously in agreement. [I91 G. HERZBERG, Infrared and Raman Spectra of Polyatomic Molecules. Van Nostrand, New York
[ll]
(1945). [20] K. J. WILMHURST,J. Chem. Phys. 2463 (1955).
11,440 (1978). [4] T. WOLFAM (editor), Inelastic Electron Tunnelling Spectroscopy. Springer-Verlag, Berlin (1978). KLEIN, A. LEGER, M. BELIN, D. DEFOURNEAU and M. J. L. SANGSTER, Phys. Rev. B7, 2336 (1973). [6] J. P. M. MAAS, Spectrochim Acta 33A, 761 (1977). [7] R. NEWMAN, J. Chem. Phys. 20, 1663 (1952). [8] T. L. CHARLTON and K. B. HARVEY, Canad. J.
[5] J.
Chem. 44,2717 (1966). 191 _ _ K. ITO and H. J.
BERNSTEIN. Canad.
J. Chem. 34.
B. A. MORROW and H. F. SHURVELL. Canad. J. Chem. 41. 1181 (1963). J. D.’ DONALDSON, J. F. ~
Spectrochim. Acta 20A, 847 (1964). 1121 E. SPINNER. J. Chem. Sot.(B). 879 (1967). ‘[13j C. J. H. SC~LJITE and K. &&, Siectr&him. 20A, 187 (1964).
Acta
[21] N. W. ALCOCK, V. M. TRACY and T. C. WADDINGTON,J. Chem. Sot., Dalton Trans 2243 (1976). [22] 0. I. SHKLAREVSKII, A. A. LYSYKHand I. K. YANZION, Sov. J. Low-Temp. Phys. 2, 328 (1976).
11, pp. 1015-1019.
058447539/81/111015-05$02.00/0 01981 Pergamon PressLtd.
1981
Evidence from inelastic electron tunnelling spectroscopy for vibrational mode reassignments in simple aliphatic carboxylate ions D. G. WALMSLEY, W. J. NELSON and N. M. D. BROWN School of Physical Sciences, New University of Ulster, Coleraine BT52 ISA, U.K. and
S. DE CHEVEIGN~, S. GAUTHIER, J. KLEIN and A. LACER Groupe de Physique des Solides de I’Ecble Normale Superieure, Tour 23-2, place Jussieu-75221, Paris Cedex 05, France (Receioed
18 April 1981)
Abstract-Tunnelling studies of the vibrational spectra of formate- and acetate-, including ‘soenriched acetate-, anions adsorbed on plasma-grown aluminium and magnesium oxide surfaces show that the COz- symmetric stretch mode can be clearly distinguished from the CH modes in the same region of the spectra. This CO*- mode of the formate ion is at 1456 cm-’ on aluminium oxide and at 1348 cm-’ on magnesium oxide: for the acetate ion the corresponding values are 1463 and 1445 cm-‘. Very well resolved vibrational spectra of surfaceadsorbed species are seen in inelastic electron tunnelling spectroscopy[ I-41. As part of a wider series of investigations using the technique we have examined the spectra of some simple aliphatic carboxylate ions adsorbed on metal oxides. The interpretation of these spectra has led us to reassess the usual assignments for the carboxylate moiety symmetric stretch vibrations and make firm suggestions in the cases of the formate and acetate anions. First consider the spectrum obtained when formic acid is adsorbed on plasma-grown aluminium oxide (Fig. la). Background structure at 940 cm-’ is due to aluminium oxide and at -36OOcm-’ is due to hydroxyl species present in the oxide. Both features have been extensively discussed in the literature of tunnelling spectroscopy and need not concern us here. Reaction of the acid on the surface to give the formate anion is suggested by the absence of an identifiable acid OH stretch mode and confirmed by the absence of the C=O stretch mode at 1780cm-’ and the C-OH stretch mode at -1100 cm-‘[5]. Instead we see clearly the broad CO*- antisymmetric stretch mode of the formate anion at 1580 cm-’ (Table 1). The CH stretch mode is found at 2875 cm-‘, the CH out-of-plane bend mode is strong at 1038 cm-’ and the CO,- rock mode is probably that seen at 242cm-‘. There remains only the question of which of the modes at 1370 and 1456 cm-’ is the CH in-plane bend and which is the CO,- symmetric stretch. The problem is resolved as follows. By analogy with i.r. and IETS assignments on higher carboxylates we initially assign the 1456cm-’ band as the CO,- symmetric stretch mode. When we examine the tunnel spectrum of the formate ion on a different surface, magnesium oxide, we find (Fig. lb) that the band at 1456cm-’
Table 1. IETS vibrational modes of (a) HCOO- on plasma-grown aluminium oxide, (b) HCOO- on plasmagrown magnesium oxide and (c) DCOO- on plasma-grown aluminium oxide
(4
@I
(cl
242
236
256 887
1038
1046
1016
1348 1370 1456 1580
1575
2724 2875 2991
2709 2815 2976
1331 1452 1589 2162 2885 2950
Assignment CO*- rock CD out-of-plane bend CH out-of-plane bend I (+ CD in-plane bend?) (CO,- symm. stretch \ +Cfi in-plane bend CH in-mane bend COz- &mm. stretch CO*- antisymm. stretch CD stretch CH in-plane bend overtone CH stretch
has disappeared but the 1370cm-’ band has grown in strength and is slightly shifted down in energy to 1348 cm-‘. We do not expect to find substantial shifts in the internal modes of the adsorbed anion simply as a consequence of a different local molecular environment; only van der Waals forces are involved and these alone do not cause noticeable shifts, for example, in bulk liquid samples (although, of course, electrostatic and hydrogen bonding interactions do). However, at the actual site of chemisorption the dominant interaction occurs between the adsorbed anion and a cation of the local oxide surface. We may anticipate a frequency shift in the modes of the carboxylate moiety of the anion since the carbon-oxygen bond characteristics will alter with different oxygenmetal cation interactions on the two different oxide surfaces. The CO,- symmetric stretch mode has an associated oscillating dipole moment per1015
D. G.
1016
WALMSLEY et al.
N D
3
N
T)
0
800
1600
LLUU c
3‘“”
me’
Fig. 1. Tunnel spectra of (a) HCOO- on aluminium oxide, (b) HCOO- on magnesium oxide and (c) DCOO- on aluminium oxide.
pendicular to the surface and is therefore likely to be more sensitive to the adsorbate-adsorbent interaction than the CO,- antisymmetric stretch mode with its nett dipole oscillating parallel to the surface. It is therefore plausible to conclude that the CO,- symmetric mode has shifted down to coincide with the original undisturbed CH mode at - 1370 cm-’ or to fall slightly below it when the acid is adsorbed on magnesium oxide. The shift is in the correct sense when the more ionic nature of magnesium oxide, as compared with aluminium oxide, is recognized. (A barely discernable downward shift amounting to -5 cm-’ is found for the CO*- antisymmetric mode.) Complementary support for the identification comes from the spectrum of deuterated (‘H2)
formic acid adsorbed on aluminium oxide (Fig. lc). Here the band at 1370 cm-’ which we attributed to the CH in-plane bend mode in the undeuterated species is much reduced in intensity. The persistence of this band is a consequence of the residual undeuterated formic acid present in the deuterated sample used (nominally -98%). The apparently high relative intensity in this case arises from the intrinsically greater interaction matrix elements expected for C-H modes compared with C-2H. It is not expected to disappear completely because some exchange is inevitable between the deuterium and surface protons; a residual CH stretch band bears witness to this process. In addition, both supposed carboxylate modes are higher in energy by approximately the same
Inelastic electron tunnelling spectroscopy amount (-20cm-‘) when compared with the undeuterated species. MAAS[6] recently summarized previous i.r. work on sodium formate and notes the challenge of assigning the CH in-plane bend and CO,- symmetric stretch modes. Studies of monocrystals with polarized radiation[7,8] led to the lower energy mode being attributed to the CH bend and the upper to the CO,- symmetric stretch as suggested by the tunnelling results; opposite attributions were made by other authors[9-121 using more standard techniques with polycrystalline samples. These and other i.r. studies of the formate ion[l3,14] show a narrow band near 1370 cm-’ but there is always a very broad band in the 145&1700 cm-’ region which clearly embraces the CO,- antisymmetric mode and may also include the band we have identified as CO,- symmetric stretch. The breadth of this band is not set by sampling considerations or instrument resolution which, for these samples, can be as good as 3, or even 1 cm-‘, but is a real linewidth. The broad band observed clearly results from a combination of thermal and local environmental contributions such as hydrogen-bonding. The tunnel spectra are taken with the samples at low temperature (2 K) to obtain sufficient instrument resolution and this low sample temperature may explain why the bands are better resolved than in i.r.; i.r. measurements on formates at low temperatures would be of interest. JONES and McLAREN[IS] found better resolution of acetate i.r. spectra at 80 K. The tunnel spectra of the propanoate and benzoate ions adsorbed on aluminium oxide and magnesium oxide have been compared in an earlier paper[l6]. They too show a substantial downward shift in energy of one band which we identify as the CO,- symmetric stretch mode. The shift is much smaller in both than observed in the formate. For propanoate ion the band moves from 1444cm-’ on aluminium oxide to 1417 cm-’ on magnesium oxide while for benzoate ion the corresponding positions are 1428 and 1396 cm-‘. A more extensive check by tunnelling has now been made on the acetate ion. Its spectrum when adsorbed on aluminium oxide may be seen in Fig. 2a. The COZ- antisymmetric stretch mode is seen clearly at 1583 cm-‘. Following the procedure of the formate case we attribute the band at 1463 cm-’ to the COZ- symmetric mode with a CH, deformation at 1419cm-‘. On a magnesium oxide surface this supposed CO,- symmetric mode has moved down in energy to 1445 cm-‘. The position of the peak of the CO*- antisymmetric mode has also apparently moved down from 1583 to 1565 cm-’ but this is a broad band and the position and width of the base of the band are the same (to within 5 cm-‘) in both spectra and we therefore regard it as not having shifted to a significant extent. Interestingly too the band at 689cm-’ on aluminium oxide which we earlier identified [ 171 as
1017
CO,- symmetric deformation also shows a downward movement to 662 cm-’ on magnesium oxide. All other bands are at substantially the same energy on both surfaces (Table 2). Further confidence in the assignment of the 1463 and 1419cm-’ bands comes from the spectra of CD,COO- and CF,COO- adsorbates on aluminium oxide[l7]. The 1419cm-’ is entirely absent from both as would be expected if it is due to a CH, mode. Final confirmation of the identification of the CO,- symmetric stretch mode comes in convincing fashion from a study of the spectrum of “Oenriched acetate ion on aluminium oxide. The spectrum is shown as Fig. 2c and peak positions are recorded in Table 2. The most notable change on going from I60 to I80 acetate involves the 1463 cm-’ peak, which is shifted down into the 1419 cm-’ peak: the result is a peak at 1403 cm-’ (-4.3%). The sensitivity of the 1463 cm-’ mode to change in the oxygen mass clearly identifies it as the C02stretching mode. The 1419 cm-’ peak, insensitive to the mass change, is therefore attributed to the CH3 deformation mode. Consistently, the other modes involving oxygen atoms are also shifted: the CO,- antisymmetric stretch from 1597 to 1573 cm-’ (-1.5%) and the CO,- scissor mode from 678 to 653 cm-’ (-3.8%). These relative shifts[l8] are somewhat smaller than those observed for the symmetric stretch, as expected for a nonlinear triatomic unit [19]. On the other hand, the modes involving the CH, group are essentially unaffected, although the separation between the two CH, rocking modes near 1040cm-’ increases slightly. The CH, deformation mode at 1419 cm-’ is hidden by the new 1403 cm-’ peak. The CO,- rocking modes at 468 and 615 cm-’ involve movement of the CH, moiety with respect to the CO2 moiety and are unaffected. Table 2. IETS vibrational modes of (a) CH$OOon plasma-grown aluminium oxide (b) CH$ZOO- on plasmagrown magnesium oxide and (c) ‘*O-enriched CH,COO- on plasma-grown aluminium oxide (a)
(b)
407 426 468 471 618 615 6891678 662 945 938 1024 1023 1051 1046 1341 1343 1372(sh) 1419 1417 1463 1445 1583/1597 1565 2866 2865 2914 2926 2964 2977 3lnM 3007 -3640 -3650
(c) 403 468 605 653 919 1016 1048 1339 1403 1573 2919 2968 3000 I
Assignment
-
COz- in-plane rock CO*- out-of-plane rock CO,- symm. def. C-C stretch CH2 rock CHg rock CH, def. CH3 def. CO*- symm. stretch CO?,- antisymm. stretch CH stretch OH stretch
1018
D. G. WALMSLEY et al.
0
800
1600
c m-’
2LOO
3200
Fig. 2. Tunnel spectra of (a) CH,COO- on aluminium oxide, (b) CH#ZOO- on magnesium oxide and (c) ‘*O-enriched CH#ZOO- on aluminium oxide.
The analysis of the effects of changing the mass of the oxygen atoms in acetic acid clearly shows that the 1463 cm-’ mode involves the movement of oxygen. We therefore assign it to the symmetric stretch of COz-, and the mode at 1419cm-’ to the CH, deformation. In careful i.r. studies of sodium acetate in which the samples were cooled to 80 K to improve resolution JONES and McLAREN[IS] assigned the COzsymmetric stretch as 1408 cm-‘. Shortly after, WILMHURST[~~] argued, by analogy with related compounds, that a better assignment would be the CO,- symmetric stretch at 1422cm-‘. More recently ALCOCK, TRACY and WADDINGTON[21] found the CO,symmetric stretch in
Al(CH,COO-), at 1465 cm-’ which is in excellent agreement with the tunnelling data. In view of the consistent behaviour seen in all the tunnel spectra with a variety of carboxylates chemisorbed on two different oxides there is little room for ambiguity in the interpretation. A satisfying feature of the assignments we propose here is the relative constancy of the CO,- symmetric stretch mode in all the aluminium carboxylates, down to the lowest member of the series. Our conclusions are strongly supported by the earlier tunnelling work of SHKLYAREVSKIIef al. [22] who studied formic and acetic acids, both physisorbed and chemisorbed, on aluminium and magnesium oxides.
Inelastic electron tunnelling spectroscopy Acknowledgements-This work has been supported by CNRS, SRC, Northern Ireland Department of Education and Imperial Chemical Industries Ltd.
REFERENCFS P. K. HANSMA, Phys. Rep. 3OC, 145 (1977). 121 _ W. H. WEINBERG, Ann. Rev. Phvs. Chem. 29. 115 (1978). [3] P. K. HANSMA and J. KIRTLEY, Accts. Chem. Res. [l]
[14] Y.
NOTO,
K.
Trans Faraday
1019 FUKUDA, T. ONISHI and K. TAMARU,
Sot. 63, 2300 (1967).
[15] L. H. JONES and E. MCLAREN, J. Chem. Phys. 22, 1796 (1954). [16] D. G. WALMSLEY, W. J. NELSON, N. M. D. BROWN and R. B. FLOYD. Aool. Surf. Sci. 5. 107 (1980). [17] N. M. D. BROWN,’ R. i. FLOYD and d. G. WALMSLEY,J. Chem. Sot., Faraday Trans II 75, 17 (1979). [18] The CO*- antisymmetric
170 (1956). [lo] K. B. HARVEY,
stretch mode gives a broad band with its peak position rather sensitively dependent on the details of sample preparation, in particular the precise conditions of oxide growth. The two values (1583/1597 cm-‘) listed in Table 2 for this mode represent distinct reproducible results obtained in our two laboratories. The shifts observed also are reproducible. Like comments apply to the CO*- symmetric deformation mode at 689/678 cm-’ but the CO,- svmmetric stretch and all other modes of the adsorbed anion are unambiguously in agreement. [I91 G. HERZBERG, Infrared and Raman Spectra of Polyatomic Molecules. Van Nostrand, New York
[ll]
(1945). [20] K. J. WILMHURST,J. Chem. Phys. 2463 (1955).
11,440 (1978). [4] T. WOLFAM (editor), Inelastic Electron Tunnelling Spectroscopy. Springer-Verlag, Berlin (1978). KLEIN, A. LEGER, M. BELIN, D. DEFOURNEAU and M. J. L. SANGSTER, Phys. Rev. B7, 2336 (1973). [6] J. P. M. MAAS, Spectrochim Acta 33A, 761 (1977). [7] R. NEWMAN, J. Chem. Phys. 20, 1663 (1952). [8] T. L. CHARLTON and K. B. HARVEY, Canad. J.
[5] J.
Chem. 44,2717 (1966). 191 _ _ K. ITO and H. J.
BERNSTEIN. Canad.
J. Chem. 34.
B. A. MORROW and H. F. SHURVELL. Canad. J. Chem. 41. 1181 (1963). J. D.’ DONALDSON, J. F. ~
Spectrochim. Acta 20A, 847 (1964). 1121 E. SPINNER. J. Chem. Sot.(B). 879 (1967). ‘[13j C. J. H. SC~LJITE and K. &&, Siectr&him. 20A, 187 (1964).
Acta
[21] N. W. ALCOCK, V. M. TRACY and T. C. WADDINGTON,J. Chem. Sot., Dalton Trans 2243 (1976). [22] 0. I. SHKLAREVSKII, A. A. LYSYKHand I. K. YANZION, Sov. J. Low-Temp. Phys. 2, 328 (1976).