8. P. R. Young, B. A. Stein, and A. C. Chang, 28th National SAMPE Symposium, 824 (1983).
9. J. A. Davies and A. Sood, Am. Lab. ll, 122 (1984). 10. K. N. Mehrotra and R. Kachwaha, Tenside Detergents 19, 92 (1982).
11. R. M. Ianniello, H. J. Wieck, and A. M. Yacynych, Anal. Chem. 55, 2067 (1983).
12. K. Nakamoto, The Infrared Spectra of Inorganic Coordination Compounds, (Wiley, New York, 1963), p. 197.
Vibrational Analysis of Terephthalate and Terephthalate-d4 Ions
F. J . B O E R I O * a n d P . G . R O T H
Department o/Materials Science and Engineering, University of Cincinnati, Cincinnati, Ohio 45221
Infrared and Raman spectra of the terephthalate and terephthalate-d4 ions have been obtained. Normal coordinate analysis was carried out for both ions by transferring force fields from previous work on aromatic esters and p-nitro-benzoate and then refining all the force constants for the carboxylate groups and the diagonal force constants for the benzene rings by a first-order perturbation technique. An interpretation of the observed spectra has been proposed and selected eigenvectors have been presented. Index Headings: Infrared; Vibrational analysis; Raman spectroscopy.
INTRODUCTION
Reflection-absorption infrared spectroscopy (RAIR) has been used extensively for investigating the adsorp- tion of organic compounds onto the surfaces of metals. For example, Boerio and Chen, 1 Allara and Swalen, 2 and Allara and Nuzzo 3 have all investigated the structure of films formed by fatty acids adsorbed onto or transferred to metal substrates by the Langmuir-Blodgett tech- nique. Chollet 4 considered the adsorption of stearamide onto copper, and Debe 5 investigated adsorption of phthalocyanine and perylene red onto copper.
One of the most important aspects of RAIR concerns the surface selection rules. Greenler showed that the electric field vector in RAIR is perpendicular to the sur- face of the substrate. 6 As a result, vibrational modes having transition moments perpendicular to the surface are much more intense in RAIR spectra than are modes with transition moments parallel to the surface. RAIR can be used to determine the orientation of adsorbed molecules, provided that the eigenvectors are known.
Very recently, there has been a great deal of interest in surface-enhanced Raman scattering (SERS), an ex- tremely surface-sensitive technique that has been used to investigate numerous organic compounds adsorbed onto substrates such as silver, copper, and gold. Selec- tion rules for Raman scattering near metal surfaces have been derived by Moskovits. ~ He showed that the most intense Raman lines will be observed for vibrations that belong to the same representation as a~z, where Z is along the surface normal. The next most intense Raman lines should be those belonging to the same representations as axz and ay z. The least intense Raman lines are in the
Received 22 September 1986. * Author to whom correspondence should be sent.
same representations as axx, O~yy, a n d Ogxy. The surface selection rules for Raman scattering are obviously not as stringent as those for RAIR. Nevertheless, they may still be used to infer the orientation of adsorbed mole- cules.
The surface selection rules for inelastic electron tun- neling spectroscopy (IETS) are somewhat similar to those for RAIR. s Modes having transition moments par- allel to the surface should be considerably less intense than modes having transition moments perpendicular to the surface. As a result, IETS can also be used to deter- mine the orientation of molecules adsorbed onto the sur- faces of metals.
Recently, we have been using surface-enhanced Ra- man scattering (SERS) and reflection-absorption in- frared spectroscopy (RAIR) to investigate the adsorp- tion of polyesters and model compounds, such as terephthalic acid and terephthalic acid-d4, onto silver island films. These compounds, and other substituted benzoic acids, usually adsorb as benzoate ions. In order to use the surface selection rules effectively and to de- termine the orientation of the adsorbed molecules, it is essential to have available accurate band assignments and knowledge of the eigenvectors for the benzoate ions.
Arenas and Marcos 9 investigated the infrared and Ra- man spectra of the phthalate, isophthalate, and tere- phthalate ions and discussed the band assignments. However, they did not carry out the vibrational analysis and they did not report the spectra of the deuterated ions. In this paper, we report the infrared and Raman spectra and the normal coordinate analysis of the TA and TA-d4 ions. The RAIR and SERS spectra of benzoic acid, p-hydroxybenzoic acid, terephthalic acid, and tere- phthalic acid-d4 will be described in a subsequent pa- per. lo
EXPERIMENTAL
Terephthalic acid and terephthalic acid-d4 were ob- tained from Aldrich Chemical Company. We prepared the dipotassium salts by neutralizing the acids with KOH, using methods described by Arenas and Marcos. 9 Stoichiometric amounts of 0.25 N KOH were added to weighed amounts of the acids with the use of a buret. After the acids were dissolved, the solutions were treat- ed with activated charcoal, filtered, and evaporated un- der vacuum to precipitate the salts. The salts were dried
/ ' \ FIG. 1. Internal coordinates used in the normal coordinate analysis of the terephthalate and terephthalate-d, ions.
in a vacuum oven at 40°C to completely remove residual water.
The infrared spectra were obtained with a Perkin- Elmer 1800 Fourier transform-infrared spectrophotom- eter. Raman spectra were recorded on a Raman spectrometer equipped with a Spex 1401 double mon- ochromator, a Spectra-Physics 165 argon-ion laser, and a Harshaw photon counting detection system.
VIBRATIONAL ANALYSIS
A planar model with D~h symmetry was assumed for both ions. The distribution of the normal modes among the irreducible representations for this model is 8 Ag + 3 B i g + 4 B2~ + 8 B3g + 3 A, + 8B1, + 8B2, + 5B3,. Modes belonging to the Ag, B~g, B2g, and B3g represen- tations are active in the Raman spectra while those be- longing to the representations BI,, B2,, and B3, are in- frared active. Modes belonging to the A, representation are inactive.
The lengths of the CC and CH bonds in the benzene rings and the CC bond between the ring and the car- boxylate group were assumed to be 1.397, 1.084, and 1.51 /~, respectively, as in our previous work on the vibra- tional analysis of aromatic esters." On the basis of the vibrational analysis of p-nitrobenzoate reported by Ernstbrunner et al.) 2 we estimated the length of the CO bond to be 1.20 A. All of the bond angles were assumed
J i
' I I I , I ' t i I ' I i I , t 1 0 0 0 ! O00 1 4 0 0 1 ~ ' 00 1 0 0 0 I100 e O 0 4 0 0 2 0 0
C M ' I
FIG. 3. Raman spectra of (A) dipotassium terephthalate and (B) di- potassium terephthalate-d4.
to be 120 ° . The internal coordinates are illustrated in Fig. 1 and are mostly self-explanatory, except for the torsional coordinate about the CC bond between the ring and the carboxylate group, which was defined as the normalized sum of the two possible trans torsions about that bond.
Initial force constants for the benzene rings were transferred from our work on aromatic esters. 11 Force constants for the carboxylate group were transferred from the work of Ernstbrunner et al. on p-nitroben- zoate? 2 Diagonal force constants for the ring and all force constants for the carboxylate group were adjusted with the use of a procedure that has been described previ- ously. ~3 The off-diagonal force constants for the benzene ring were kept fixed.
Ag - 303 c m - 1 A g - 8 5 1 c m - 1
1800 16~
{ 1488 1206
J
f- r
i
B
1060 880 Cn-t 600
FIG. 2. Infrared spectra of (A) dipotassium terephthalate and (B) dipotassium terephthalate-d4.
Ag - 1 6 0 0 ¢ m - 1
A g - 1 1 7 1 c m - 1
FIG. 4. Selected eigenvectors from the vibrational analysis of tere- phthalic acid.
464 Volume 41, Number 3, 1987
TABLE I. Observed and calculated frequencies and potential energy TABLE I. Continued. distribution for TA and TA-d4. ~.'
Observed Calculated Potential energy distribution (%)
Observed frequencies in parentheses were not used in the force con- stant refinement. Since the contribution of off-diagonal force constants to the potential energy distribution may be negative, the sum of the contributions by diagonal force constants may exceed 100%.
The vector of force constants • was refined by the first-order perturbation technique with the use of the equation
A~ = (J+WJ)-IJ+WAA (1)
where A4~ is a vector of corrections to the force con- stants, J is the Jacobian, W is a weight matrix where Wij = 5ij/h~, and AA = Ao - Ac where Ao and At are vectors of observed and calculated eigenvalues. The cor- rections were added to • and the procedure was repeat- ed until the calculated eigenvalues converged toward those observed.
Matrix J+WJ usually tended toward singularity. That tendency was controlled with the use of the method re- ferred to as damped least-squares. TM We added a damp- ing factor equal to 0.001 to each diagonal element of (J÷WJ) before it was inverted, to calculate the correc- tions to the force constants as described above.
RESULTS AND DISCUSSION
The infrared and Raman spectra of the TA and TA- d4 ions are shown in Figs. 2 and 3. The calculated and observed frequencies and the potential energy distri- butions are summarized in Table I, and the final force
APPLIED SPECTROSCOPY 465
TABLE II. Valence force constants for terephthalate and terephthal- ate-d4 ions. ~.b
Coordi- Force nate(s) Atom(s) Assumed Calculated
constant involved common value value
Kx X KR R KT T K¢ 7r
H0 0 H¢ 4~ H, H~ Hz Z H, HM M n~ r H M, M' FT ° T, T FT ~ T, T FT P T, T FT~ T, ¢ FT~ T, 4~ F~ ° ¢, ¢ F~ ~ ¢, ¢ F¢ P ¢, ¢ FrR T, R FT~ T, fl FR. R, FMz m M, Z FMZ ° M, Z F,z ° ~, Z F,z ~ ~, Z f o ~,
f~P #, fM P M, M fz ° Z, Z f,M ° ~, M f.M m #, M Fxx X, X FRx R, X Fx. x, a Fx8 x, 0 F~ R, t~ F~ R, a FR, R, Fx~ (~) X, T FXT (~) X, T
0.0240
C 0.7716 -0.3189
0.2895 C,CC 0.0918 C,CC 0.1860
b 0.0001 b -O.OO52 b 0.0044 C 0.2506
CC 0.1187 CC 0.5990 b 0.0281 b -0.1589 b -0.1589 b 0.0281 b -0.0723 b -0.0003 b -0.0143 b -0.0143 b -0.0579 b -0.0723 b -0.0003 C C
Units are md/A for stretching force constants, md/rad for stretch- bend interactions, and md - A/(rad) 2 for bending constants.
b For expression "b" in this table, see Ref. 15 for a more complete description of these force constants.
constants are summarized in Table II. Several of the eigenvectors for the TA ion are shown in Fig. 4.
Most of the assignments for the TA ion are similar to those proposed by Arenas and Marcos2 However, there are a few differences. For example, Arenas and Marcos assigned the medium intensi ty band near 638 cm -I in Raman spectra of the TA ion to an OCO deformat ion mode belonging to the Ag representa t ion and a weak shoulder on the high-frequency side of the same band to the B3~ ring bending mode. However, a prominent band is always observed near these frequencies in Ra- man spectra of p - subs t i tu ted benzene derivatives tha t do not have carboxylate g roups- - such as dimethyl t e rephtha la te n and p - x y l e n e ~ - - a n d is assigned to the Bag ring bending mode. The same assignment is pre- ferred here.
Reference to the potent ial energy distr ibution indi- cates tha t the OCO bending mode belonging to the Ag representat ion is strongly coupled with other modes such as ring deformat ion and stretching. We have assigned tha t mode to the weak band near 709 cm -I in the Raman spectra of the TA ion. Arenas and Marcos 9 assigned the same band to a CC out-of-plane bending mode in the B2g representat ion. The assignment tha t we have made is suppor ted by the normal coordinate analysis and by the Raman spectra of the TA-d4 ion. The calculations show tha t the OCO bending mode should not shift sig- nificantly upon deuterat ion, and a weak band is ob- served near 688 cm -1 in the Raman spectra of the TA- d4 ion, representing a shift of about 21 cm -1. The normal coordinate analysis shows tha t the B2g modes are strong- ly coupled and tha t the CC out-of-plane bending mode should be assigned to the medium intensi ty band near 257 cm-L
Arenas and Marcos 9 assigned the strong bands near 823 and 745 cm -1 in the infrared spectra of the TA ion to a CH out-of-plane bending mode in the B3, represen- ta t ion and a ring bending mode in the B1, representa- tion, respectively. However, results obtained from the normal coordinate analysis and the infrared spectra of the TA-d4 ion indicate tha t those assignments should be revised. According to the calculations, the B3, CH out- of-plane bending mode should shift downward in fre- quency about 117 cm -~ upon deuterat ion; a strong band is, in fact, observed near 641 cm -1 in the infrared spectra of the TA-d4 ion, represent ing a shift of about 104 cm '. The band near 823 cm -1 is assigned to a B1, mode in- volving mostly carboxylate stretching and bending. T h a t mode should shift by approximately 54 cm -~ in deuter- ation and is assigned to the strong band near 790 cm -~ in the infrared spectra of the TA-d4 ion.
According to the normal coordinate analysis, there should be three bands with frequencies between 400 and 500 cm -~ in the infrared spectra of bo th TA and TA-d4 ions. These bands are associated with vibrational modes tha t involve stretching of the CC bond between the ring and the carboxylate group and bending of the COO an- gle (B1,), torsion about the ring CC bonds and out-of- plane bending of the CC bond between the ring and the carboxylate group (B~), and COO rocking (B2,). Three bands were observed near 507, 500, and 453 cm -1 in the infrared spectra of the TA ion, bu t only two were ob- served, near 486 and 410 cm -1, for the TA-d4 ion. The strong bands near 507 and 500 cm ~ in the infrared spec- t ra of the TA ion were assigned by Arenas and Marcos to the B3, out-of-plane bending mode, and the medium intensi ty band neeir 453 cm -~ was assigned to the B2, CO0 rocking mode?
We have considered two interpretat ions of these bands. In the first, the bands near 453 and 410 cm 1 in the spectra of TA and TA-d4 ions were assigned to the B3, out-of-plane bending mode. The bands near 507 and 486 cm -1 for TA and TA-d4 ions were assigned to the B1, mode associated with stretching of the CC bond between the ring and the carboxylate group and bending of the CO0 angle. The band near 500 cm -1 in spectra of the TA ion was assigned to the B2u COO rocking mode, and it was assumed tha t the corresponding band in the spec- t ra of the TA-d4 ion was not observed.
466 Volume 41, Number 3, 1987
In the second case, the bands near 500 and 410 cm -1 in spectra of TA and TA-d4 ions were assigned to the B3, out-of-plane bending mode, and the bands near 507 and 486 cm -1 were assigned to the B~u mode involving stretching of the CC bond between the ring and the carboxylate group and CO0 bending. The band near 453 cm-' in the TA ion was assigned to the B2, CO0 rocking mode, and it was assumed that the corresponding mode in the spectra of the TA-d4 ion was not observed.
The first interpretation of the bands between 400 and 500 cm-' resulted in the minimum squared deviation between calculated and observed frequencies, and those assignments were used in the final calculations. How- ever, until more information is available, the second set of assignments cannot be completely excluded.
Arenas and Marcos s assigned weak bands near 1439 and 1352 cm -1 in the infrared spectra of the TA ion to modes in the B2u representation related to ring stretch- ing and CH in-plane bending. The calculated frequen- cies for these modes are 1401 and 1284 cm -~ for the TA ion and 1335 and 1273 cm -~ for the TA-d4 ion. We have assigned the highest frequency mode to the weak band near 1439 cm -~ in infrared spectra of the TA ion, as suggested by Arenas and Marcos, and the lowest fre- quency mode to a weak band near 1288 cm-L The lowest frequency mode should hardly shift upon deuteration and is assigned to a weak band near 1330 cm -1 in the infrared spectra of the TA-d4 ion. The highest frequency mode should shift downward by about 70 cm -~ upon deuteration, but its location in the infrared spectra of the TA-d4 ion is probably obscured by the very strong and broad band near 1350 cm -~ that is associated with the symmetric stretching mode of the CO0- group.
It is very interesting to note that there is one strong band near 858 cm -~ in the Raman spectra of the TA ion that is assigned to the Ag ring breathing mode. However, there are two strong bands, near 870 and 838 cm -~, in the same region in the spectra of TA-d4. The calcula- tions show that the CH in-plane bending mode belong- ing to the A~ representation shifts from about 1173 cm-' to 870 cm -~ upon deuteration and acquires some of the character of the ring breathing mode. That change in
potential energy distribution accounts for the increased intensity of the Ag in-plane bending mode in the Raman spectra of TA-d4.
Finally, as discussed above, the most intense lines in Raman spectra of molecules adsorbed onto metal sur- faces should be observed for vibrational modes belong- ing to the same representation as azz (the Ag modes for TA). It is also frequently considered that the strongest lines in SERS spectra involve motions that are largely perpendicular to the surface. The most intense lines ob- served in the SERS spectra of TA are near 1620, 1143, 868, and 392 cm -110 and correspond to the A~ modes calculated at 1600, 1125, 851, and 303 cm -1. Reference to Fig. 4 indicates that all of these modes involve mo- tions perpendicular to the surface when TA is adsorbed with a vertical conformation. However, the Ag mode cal- culated at 1171 cm -1 also involves motion perpendicular to the surface but is not observed in the SERS spectra of TA, 1° indicating that the selection rules for Raman scattering near metal surfaces must be applied with care to SERS spectra.
ACKNOWLEDGMENTS
This research was supported in part by grants from the 3M Com- pany and from the National Science Foundation Polymers Program. The assistance of Kristen A. Boerio in preparing the figures is also acknowledged.
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