J . Phys. Chem. 1992, 96, 5284-5290
5284
in GeH3F--HF, the H F stretching mode occurs at 3677 cm-', whereas in SiH,F--HF, the v, mode occurs at 3809 cm? therefore GeH3F is a stronger base than SiH3F. In other words, there is more electron density on the F atom of the GeH3F submolecule to provide for a stronger interaction with HF. This follows from the increase in electropositive nature of Ge as compared to Si. Germane itself interacts slightly more strongly with HF than silane in this regard.6 Finally, SiF was produced whereas GeF was not, and the yield of SiF, was substantially greater than the yield of GeF,. This can be accounted for by the fact that an SiF bond is energetically stronger and a higher degree of fluorination was observed for SiH4 than for GeH, in these experiments.
Acknowledgment. We gratefully acknowledge financial support from N.S.F. Grant C H E 88-20764.
References and Notes (1) Wilde, R. E.; Srinivasan, T. K. K.; Harral, R. W.; Sanker, S. G. J . Chem. Phys. 1971, 55, 5681. (2) Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1970, 52, 2594. (3) Smith. G. R.: Guillorv. W. A. J . Chem. Phvs. 1972. 56. 1423. (4) Withnall, R.; Andreis, L. J . Phys. Chem. 1985,89, 3261. ( 5 ) Withnall, R.; Andrews, L. J . Phys. Chem. 1990, 94, 2351. (6) Davis, S. R.; Andrews, L. J. Chem. Phys. 1987, 86, 3765. (7) Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1968, 49, 4269. (8) Milligan, D. E.; Jacox, M. E.; Guillory, W. A. J . Chem. Phys. 1968, 49, 5330. (9) Hastie, J. W.; Hauge, R.; Margrave, J. L. J . Phys. Chem. 1968, 72, 4492. (10) Huber, H.; Kundig, E. P.; Ozin, G. A,; Vander Voet, A. Can. J . Chem. 1974, 55, 95. (1 I ) Fredin, L.; Hauge, R. M.; Kafafi, Z. H.; Margrave, J. L. J . Chem. Phys. 1985, 82, 3542.
(12) Yamada, C.; Hirota, E. Phys. Reu. Letr. 1986, 56, 923. (13) Yamada, C.; Kanamori, H.; Hirota, E.; Nishiwaki, N.; Itabashi, N.; Kato, K.; Goto, T. J. Chem. Phys. 1989, 91, 4582. (14) Hudgens, J. W. J. Chem. Phys. 1991, 94, 5331. (15) Jacox, M. E. Chem. Phys. 1979, 42, 133. (16) Johnson, G. L.; Andrews, L. J. Am. Chem. SOC.1980, 102, 5736. (17) Andrews, L.; Johnson, G. L. J . Chem. Phys. 1982, 76, 2875. (18) Johnson, G. L.; Andrews, L. J. Am. Chem. Soc. 1982, 104, 3043. (19) Andrews, L.; Johnson, G. L. J . Phys. Chem. 1984, 88, 425. (20) Ayers, G. P.; Pullin, A. D. E. Spectrochim. Acta, Part A 1976, 32, 16220. (21) Redington, R. L.; Milligan, D. E. J . Chem. Phys. 1962, 37, 2162. (22) Jones, L. H.; Swanson, B. I.; Ekberg, S.A. J . Chem. Phys. 1984,81, 5268. (23) Arkell, A. J. Am. Chem. SOC.1965, 87, 4057. (24) Dupuis, M.; Watts, J. D.; Villar, H. 0.;Hurst, G. J. B. Comput. Phys. Commun. 1989, 52,415. (25) Dupuis, M.; Phys, J.; King, H. F. J . Chem. Phys. 1976, 65, 111. (26) Newman, C.; OLoane, J. K.; Polo, S. R.; Wilson, M. K. J. Chem. Phys. 1956, 25, 855. (27) Robiette, A. G.; Cartwright, G. J.; Hoy, A. R.; Mills, I. M. Mol. Phys. 1971, 20, 541. (28) Andrews, L. J. Phys. Chem. 1984,88, 2940. (29) Freeman, D. E.; Rhee, K. H.; Wilson, M. K. J . Chem. Phys. 1963, 39, 2908. (30) Sahu, S. N.; Shi, T. S.; Ge, P. W.; Corbett, J. W.; Hiraki, A,; Imura, T.; Tashiro, M.; Singh, V. A. J . Chem. Phys. 1982, 77, 4330. (31) Ismail, Z. K.; Fredin, L.; Hauge, R. H.; Margrave, J. L. J. Chem. Phys. 1982, 77, 1626. (32) Huber, K. P.; Herzberg, G. C o n " ojDiatomic Molecules; Van Nostrand Reinhold: New York, 1979. (33) Cotton, F. A.; Wilkinson, G. Advanced Znorganic Chemistry, 4th ed.; John Wiley and Sons: New York, 1980. (34) Agrawalla, B. S.; Setser, D. W. J . Chem. Phys. 1987, 86, 5421. (35) Agrawalla, B. S.; Setser, D. W. Gas Phase Chemiluminescence and Chemiioniration; Fontijn, A., Ed.; Elsevier Science Publishers: Amsterdam, 1985.
Infrared Spectra and Structure of Acetylene on NaCI( 100) S.Keith Dunn and George E.Ewing* Department of Chemistry, Indiana University, B loomington, Indiana 47405 (Received: November 26, 1991; In Final Form: March 10, 1992)
Infrared spectra of C2H2and C2D2on NaCl(100) are reported. Isotherms are presented for 78,84, and 95 K. The isosteric heat of adsorption for 8 = 0.5 is -30 f 2 kJ/mol, while the interaction energy between adsorbates is -4 kJ/mol. The v3 band of acetylene is split into three peaks. Multiple peaks for this nondegenerate vibration result from both correlation-field and static-field splittings. The spectra are qualitatively explained by a bilayer structure similar to 1.5 layers of the low-temperature crystalline phase of acetylene.
Introduction Infrared spectroscopy is useful for determining the structure of molecules adsorbed onto alkali halide surfaces.'" While diffraction methods provide information about the adlayer lattice sites, they tell little about molecular orientations. Polarized IR studies are needed to supply this information. From these IR studies, structures have been proposed for C033' and C024+6on NaCl(100). At submonolayer coverages in the high-temperature phase, CO molecules are randomly distributed among all available adsorption sites of NaCl( 100) because the adsorbates exhibit little interaction among neighbors. This ideal lattice gas gives rise to Langmuir isotherms.' However, the case of C02on NaCl is qualitatively different. Because of interactions among the adsorbed molecules, C02grows in constant-density islands, and the monolayer structure closely resembles one layer of the molecular ~ r y s t a l . ~These ,~ interactions, which lead to non-Langmuir isotherms, are successfully modeled by a molecular quadrupole-quadrupole approximation.6 Acetylene, like COz, possesses a large permanent molecular quadrupole moment, but, unlike C 0 2 ,exhibits hydrogen bonding
in condensed phases.8-22 The dominant surfaceadsorbate force for acetylene on NaCl is expected to result from electric field gradientquadrupole interactions. The adsorbate-adsorbate interactions are likely quadrupole-quadrupole and hydrogen bonding. Since the same types of forces govern the adsorbed phase, molecular crystals and van der Waals complexes, structural similarities are anticipated. In particular the characteristic T-shape between neighboring molecules in the condensed phases and clusters of acetylene is likely to O C C U ~ . ~ - ' ~ Schinzer reported spectra from acetylene adsorbed onto polycrystalline NaCl films. Since the v5 bending mode produced only one peak, he concluded that the degeneracy of this mode was not lifted upon adsorption, and therefore the molecular axis of acetylene is perpendicular to the substrate.' While the arguments used in this discussion were appropriate, the bandwidth discussed was approximately 20 cm-' and could obscure splittings of spectroscopic features. By use of a single-crystal apparatus which employs plane polarized light, it has recently become possible to obtain spectral features with full width at half maximum (fwhm) below 0.1 cm-' for small molecules adsorbed onto NaCl( 1oO)?6J33.24 In this paper
0022-3654/92/2096-5284%03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5285
Structure of Acetylene on NaCl( 100)
TI z
z
TABLE I: Spectroscopic Data for the u3 Mode of Acetylene on _-'
NaCI(100)
species
'L
(cm-')
neat C2H2 3248.8 3230.2 3227.2 neat C2D2 2412.7 2396.1 2394.6 15% C2D2 2413.1 2395.9
-t' A
tz
a
r (cm-1) 1.2 1.6 1.5 0.6 1.3 1.1 1.1 1.8
10-2A,
(cm-1) IO-~A,(cm-1)
0.7 1.40 1.5" 0.41 0.79" 0.81° 0.06 0.25
0.80 2.04 1.40 0.12 0.51
Obtained by deconvolution into Lorentzian curves. 0.008
i
c2D2 I'
0.004
1
-
t
Figure 1. Coordinate and angle definitions. The z axis is perpendicular to the surface. The x and y axes lie along the [ 1001 directions in the surface of the crystal: (a) shows the direction of propagation, k, and the electric field direction of Epand E, polarized light; (b) shows a transition dipole, p, at a tilt angle a from the surface normal and a rotation angle fi from the x axis. we report results for acetylene on NaCl using this type of apparatus. Analyses of the u3 asymmetric hydrogen stretching mode of C2H2and C2D2are used to suggest a reasonable structure. Estimates for the surfactadsorbate and adsorbate-adsorbate interactions are inferred from the adsorption isotherms.
C
e
Experimental Section The experimental details are given e l ~ e w h e r e . ~Only ~ . ~ ~information relevant to the reported results is presented here. Two 30-mm X 20-mm X 3-mm NaCl crystals were cleaved in air along the (100) plane. These slabs were mounted on a copper sample holder so that each face was at a 60' angle to the interrogating light beam. The sample holder was attached to an open-cycle liquid helium cryostat. A thermistor and resistive heater were used to provide constant temperature, with an estimated accuracy of f l K.4 Temperature was maintained to within 0.2 K. The crystals and sample holder were enclosed in an ultrahigh vacuum chamber where an ion pump (Varian) maintained a base pressure of less than 1 X lo+' mbar. Between experiments the crystals were stored at 380 K,because all atmospheric gases desorb at this temperature, but evaporation of the NaCl substrate is minimal. The vacuum chamber was fitted with wedged CaF2 windows to reduce etalon fringes. IR spectra were recorded using a Fourier transform infrared spectrophotometer (Mattson Nova Cygni 120). The light was directed through a wire grid polarizer (Molectron) and collected onto the element of a liquid nitrogen-cooled indium antimonide detector. E, polarized light has its electric field vector perpendicular to the plane of incidence (Le. parallel to the substrate), while E, light is polarized parallel to the plane of incidence and has electric field vector components perpendicular and parallel to the surface (see Figure 1). The C2H2used was atomic absorption grade (Air Products) and was reported to have a chemical purity of 99.96%. The C2D2 (Isotach) had a reported isotopic purity of 99%. The samples were condensed in a liquid nitrogen trap and distilled using an acetonedry ice slurry (-200 K). Pressure readings were obtained using an ion gauge (Vacuum Generators) and corrected for the ionization cross section of acetylene.25
Results Spactra are presented for three sets of experiments performed at 78 K: one with neat C2H2,one with neat C2D2,and the other with 15% C2D2in C2H2. Spectroscopic information is collected in Table I. Bandwidth (fwhm) values are given by I' (cm-I).
n b
0.004
0
Ill I ,
3260
3250
3240
3230
3220
z
v (cm") Figure 2. IR absorption spectra of saturated coverage of acetylene on NaC1( 100) at 78 K using E, polarized light. The C2D2spectrum was obtained under an equilibrium pressure of 5 X lo4 mbar CzD2. The CzH2spectrum was produced at 5 X lo-* mbar.
Isotherms are reported at 78, 84,and 95 K. At 78 K in the pressure range from 5 X lo+' to 5 X la-8 mbar C2H2(or C2D2)three peaks are present in the v3 stretching region of the neat samples of both isotopes. This is shown in Figure 2 for C2H2 and C2D2with E, polarization and in the upper panel of Figure 3 for C2D2with E, polarization. The spectra in Figure 2 are qualitatively similar in that both contain a high-frequency singlet and a low-frequency doublet. The splitting of the doublet in the C2H2spectrum is twice that in the C2D2spectrum, while the frequency difference between the singlet and the center of the doublet in the C2H2spectrum is nearly the same (a factor of 1.16) as in the C2D2spectrum (see Table I and Figure 2). Figure 3 shows E, spectra from neat CzD2and a mixture of 15% CzD2in C2H2. The E, spectrum of the isotope mixture is qualitatively similar. Two peaks result from C2D2as the minority isotope, as opposed to the three from neat C2D2. The high-frequency singlet remains, but the low-frequency doublet seen in the neat spectrum is a singlet for the isotopic mixture. Isotherms are presented in Figure 4. Points on the isotherms are obtained by using the integrated absorbance, defined as
z4 =
Sh"dlog,, Uo/O d7
(1)
5286 The Journal of Physical Chemistry, Vol. 96, No. 13, I992
Dunn and Ewing temperatures at pressures approximately 3 orders of magnitude below the vapor pressure of solid acetylene.26 A listing of integrated absorbances for the saturated coverages of C2H2,C2D2and the isotopic mixture is found in Table I.
0 06
-
Discussion Isotherms. The steeply rising isotherms of Figure 4 show nonideal adsorbate behavior, so a model which includes lateral interactions between adsorbates should be used. The quasichemical approximation, outlined by Hill27and first applied to an adsorption isotherm by Fowler and Guggenheim?* allows for independent pairwise interactions of nearest neighbors and provides a weighted probability that an adsorbate will have its neighboring adsorption sites occupied. The adsorption isotherm for this approximation is
A
b 0040
r b
-P --
a li
Po
C
qgas
qads
exp[DO/kr]
exp[nw/2kr] X
e L
117
-.
where y = [l
0 2
0
I
I
I
2410
400 -v 2(cm
2
2390
0
'1
Figure 3. Spectra of neat C2D2and 15% C2D2in C2H2 at 5 X lo4 mbar using E , polarization. T
T
-?-
.7..
i
...
i
I
O4
- 48(1 - e ) ( l - e ~ p [ - w / k r ] ) ] ~ / ~
The zero-point binding energy of the molecule to the surface and the interaction energy between a pair of neighboring adsorbates are given by Do and w, respectively. The number of nearest neighbors for a molecule in a adlayer is n, k is Boltzmann's constant, and T i s the temperature. For a square lattice n = 4, but neither nw nor Dois strongly dependent on the number of nearest neighbors. A value for nw is extracted from this approximation by combining qW, qab, and exp[Do/kT] into one constant, K . By varying nw and choosing K so that p = 1.2 X mbar C2H2for 8 = 0.5 we obtain a series of line shapes for the isotherm at 84 K. The best fit for the data is (nw/kTj = -6, as shown by the dashed line in Figure 4. This corresponds to a lateral interaction energy of nw = -4 kJ/mol. Using the above value for nw, we can obtain an estimate for Do if qgasand qab are known. From statistical thermodynamics we know that, for a linear molecule2'
1
O Z 1
T
I
1 10*
loa
10.
106
10
Pressure (mbar) Figure 4. Adsorption isotherms for acetylene on NaCl(100). The dashed lines represent model isotherms produced by the quasi-chemical adsorption approximation at 78, 84, and 95 K. The parameters used for this approximation are Do = 27 k 1 kJ/mol and nw = -4 kJ/mol.
Subscripted values, A,and A, represent measurements made using E, and E, polarized light, respectively. The integrated cross section, per molecule, is assumed to be constant with respect to coverage, and 8 = 1 refers to the maximum integrated absorbance observed. As seen in Figure 4, no submonolayer spectra were collected a t 78 K. At this temperature saturated coverage is mbar. A full isotherm reached at pressures as low as 1.O X is reported at 84 K for neat C2H2. While no absorbance is seen mbar, almost full a t pressures 11.1 X lo-* mbar at 1.3 X coverage is reached. Structural changes are seen in the spectra of both neat isotopes for less than full coverage. The singlet is broader, and there are as many as four peaks in the doublet region. However, bands in both the singlet and doublet regions grow in simultaneously. The isotherm at 95 K is incomplete since the maximum absorbance is never attained, but the step from no coverage to 8 = 0.7 occurs between 3.0 X and 3.1 X mbar C2H2. Saturated coverages are reached at all reported
where h is Planck's constant, and p o is the standard state pressure (1 Pa or 0.01 mbar). Except for B = 1.9 cm-I, the rotational constant for acetylene, all terms in eq 3 are in SI units. Following the development of Richardson, Baumann, and EwingZ9we express the adsorbed molecule partition function as
where c is the speed of light, and the li(in cm-I) refer to the harmonic oscillator vibrational frequencies of the five normal modes associated with movement of an acetylene molecule against the surface.. For CO on NaCl, these frequencies range from 30 to 140 Since CO and C2H2have nearly identical molecular masses, we estimate these frequencies for C2H2on NaCl to be 100 cm-I, but for completeness the error limits associated with = 50 and 200 an-'. values of Do and AHab are obtained by using lj Solving eq 2 for Do at 8 = 0.5 we get Do = kT[ In
(E) )(: -In
+ In
(%)I
(5)
With T = 84 K this gives Do = 27 f 1 kJ/mol. Using the above values for nw and Do extracted from the 84 K isotherm, eq 2 reveals the pressure that gives 8 = 0.5 at 95 K to be (3 1) X mbar C2H2. This is in agreement with our experimental results. Also, we predict that half coverage should
Structure of Acetylene on NaCl( 100) be obtained at 78 K with a pressure of (4 f 1) X 10-" mbar C2H2, below the base pressure of our vacuum system. Figure 4 shows the experimental and modeled (the dashed lines) isotherms for all three temperatures. The Clayperon equation is used to find AHads:
Solving for
mad,
and combining eqs 2-6 we get
.
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5281 splitting for neat samples of either isotopomer (see Figures 2 and 3). We offer a structural model consistent with this spectroscopic data. We consider our neat saturated systems as being made up of two sublattices. Sublattices I experiences correlation-field splitting and produces a doublet in the majority isotope, while sublattice I1 exhibits no correlation-field splitting and produces only a singlet. Determination of Molecular Tilt. The tilt angle for the molecular axis of the adsorbate can be obtained from the polarized spectra. From Berg and Ewing we know4
.
(7) for 8 = 0.5. This produces a value of AHads= -30 f 2 kJ/mol, comparable to m a d s for C02 a t 8 = 0.5, which is reported as -26 kJ/mol.6 The model employed here, which assumes that adsorbed molecules have four nearest neighbors and are on identical adsorption sites, oversimplifies the bonding in the acetylene adlayer, as we shall soon show. However, there is qualitative agreement with our experimental isotherms. Therefore, we believe the model gives gocd average values for the surface-adsorbate and adsorbate-adsorbate interactions. Vibrational Assignment. All peaks resulting from C2H2vibrations occur in the same spectral region as the v3 vibrations in the gas phase (t = 3289 ~ m - l ) , ~the I low-temperature solid phase (5 = 3226 and 3249 cm-1),17922 and the van der Waals dimer (t = 3281 and 3273 cm-').8 Peaks produced by C2D2vibrations are close to the v3 frequencies of both the gas phase (t = 2439 ~ m - ' ) ~ ' and the low-temperature solid phase (2392 cm-l).I7 The gas-phase IR spectrum of C2H2is complicated by a strong Fermi resonance between v3 and the combination mode of v2 + v4 + The Fermi resonance is broken in the solid and van der Waals complexes. As a result of the hydrogen bonding, the v3 vibration is red shifted while the combination band, which contains bending modes, is blue shifted.I7 There is no Fermi resonance involving the v3 vibration of gas-phase C2D2. We conclude that the Fermi resonance is broken upon adsorption, because three peaks are observed in the v3 regions of both C2H2and C2D2. For the above reasons all peaks are assigned to the v3 vibration of either C2H2or C2D2. There are at least two mechanisms by which a nondegenerate vibration, such as v3, can produce multiple IR signals: correlation-field splitting and static-field or environmental splitting. Correlation-field splitting occurs when a regular array of oscillators are coupled so that one nondegenerate internal molecular vibration contributes j collective modes to the array, where j is the number of molecules per unit cell.32 If two or more of the j collective modes are energetically different and infrared active, multiple signals are observed. We have examples of this splitting for C 0 2 on NaC1(100)4*6and for CO on NaC1(lOO)S in its lowtemperature phase. In both cases two molecules per unit cell describe the monolayer structure and two infrared absorptions are observed. Static-field or environmental splitting can be caused by multiple adsorption sites which perturb adsorbates' molecular vibrations differently. Environmental splitting occurs for C029and C2H2I on polycrystalline NaCl films where molecules are adsorbed to a variety of edge or corner sites as well as smooth faces of the crystallites. Two acetylene molecules with different environments producing two IR signals is also observed in the gas-phase dimer study by Fraser et a1.8 The dimer, known to be T-shaped,s-lo shows a splitting of 8 cm-I between the v, vibrations of the acetylene molecules along the leg and the top of the T.8 Both environmental and correlation-field splittings are observed for the v3 vibration of acetylene on NaCl( 100). When a mixture of 15% C2D2 in C2H2is adsorbed, the C2D2molecules have few nearby resonant oscillators with which to couple and produce only those peaks due to different adsorption sites. Spectra from this isotopic mixture show two peaks in the v3 region of C2D2,corresponding to two adsorption sites or environments (see Figure 3). The molecules on one of those sites exhibit correlation-field
where a is the angle between the transition dipole and the surface normal (see Figure 1) and q is the refractive index of the substrate. The v3 mode of acetylene has its transition dipole along the molecular axis. Therefore, eq 8 provides the tilt angle of the molecule. Since the collective modes of sublattice I are not strongly coupled to th_ose,Of sublattice 11, we consider each sublattice separately. (APIAs)= 0.47 f 0.05 for the doublet, and 0.51 f 0.05 for the singlet. Using q = 1.52 for NaCl,33eq 8 gives a1= 81 f 5 O and all = 79 f 5 O . The molecules in both sublattices are nearly, but not quite, parallel to the surface. Density of Adsorbates. By using photometric relationships, derived by Richardson et al.,24 we may obtain a value for the surface density of adsorbates, S A,(2.303) cos 0 S = 3~.,,,~N[cos~a (9) sin2 e + (y2) sin2 a cos2 61 and &(2.303)
S=
COS
O(l
+ q2)
N [ ~ sin2 B a~] ~ ~ ~
(10)
The angle of incidence for the interrogating light beam is 0, and N is the number of faces inLerrogated. The factor of 1 + q2 occurs in the expression involving A,, because E, light is partially reflected by the substrate. E , light is not reflected, because 0 is the Brewster angle for NaCl. is the integrated cross section if the molecule is randomly oriented with respect to the probing electric field, as in the gas, liquid, and polycrystalline phases. It remains to obtain a feasible value for Brand for our system. Previous studies of similar systems3v4have employed the oriented-gas model, which assumes electrical properties, such as the oscillator strength, of the gas-phase molecule are unchanged upon adsorption. This is not likely to be satisfactory for acetylene. Khanna et al. have recently shown that the oscillator strength of the low-temperature, hydrogen-bonded solid phase of acetylene is significantly larger than that of the gas phase.22 Since the strength of the hydrogen bonding in the adlayer is similar to that in the solid, we use the solid-phase integrated cross section of (2.1 f 0.6) X cm/moleculeZ2 as an estimate for B . , ~ ~ . First we consider only sublattice I. Using eq 9, with A, = 0.029 f 0.003 cm-I, 0 = 60°, a = 82O, N = 4, and Brand = (2.1 f 0.6) X cm/molecule, we get S' = (1.0 f 0.4) X 10Is molecule/cm2, which is within experimental error of the density of sodium-chloride ion pairs at the surface (6.3 X 1014molecule/ cm2).34 Since the lower limit for surface density of sublattice I is approximately the density of available adsorption sites, we will consider all surface sites to be populated with molecules in sublattice I. This means one acetylene molecule per surface ion pair. If we consider all surface sites occupied by sublattice I, then sublattice I1 must form a second layer. Since the integrated absorbance of the singlet is only one-fourth that of the doublet, we know that sublattice I1 is less densely packed than is sublattice I, or that the oscillator strength of molecules in sublattice I1 is less than that of those in sublattice I, or both. However, since the hydrogen bonding exhibited in the solid only increases the oscillator strength by a factor of 2, it is unlikely that the oscillator strength of the molecules in one sublattice is 4 times that of those
Dunn and Ewing
5288 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
in the other. Hence, the second layer is probably less densely packed than the first and does not experience hydrogen bonding between neighbors. Furthermore, if we assume that, as in the gas-phase dimer study, the molecular cross section for the bound acetylene is about twice that of the unbound,s the density of the second layer is half that of the first. Correlation-FieldSplitting. Correlation-field splitting is often modeled by transition dipole-transition dipole couplingp but higher order multipole contributions are sometimes important.35 The transition dipole, lpl, for a nondegenerate mode is related to the integrated cross section by36
We can obtain values for band1and by rearranging eq 9 and I = 6.3 X lOI4 molecules/cm2 and SI1= 3.7 X lOI4 using S molecule/cm2. This gives band1= (3.1 f 0.2) X cm/molecule and brand" = (1.5 f 0.2) x 10-l~cm/molecule, close to the gasphase cross section of 1.2 X c m / m ~ l e c u l e . ~Using ~ above values for brand' and brandt1, 3' = 3229.0 cm-', and 3'' = 3248.8 gm112cm512 cm-' and cgs units we get lplH1= (0.15 f 0.1) X s-', or 0.15 f 0.1 D, and lp(H1l= 0.11 f 0.1 D. Following the same method for the deuterated isotope we get IdD'= 0.13 f 0.1 D and (pID" = 0.09 f 0.1 D. If we assume the correlation field splitting displayed by sublattice I is due only to transition dipole-transition dipole coupling and follow the development of H e ~ t e used r ~ ~by Berg and Ewingp we see that the magnitude of the splitting is as follows: A Z = (1pI2/hc)S3
Figure 5. Expected adsorption site for first layer (sublattice I) of acetylene on NaCl(100). The small and large circles represent Na+ and C1ions, respectively. The end of the molecule with the checked pattern is tilted up from the surface.
" avb C
C;s
(12)
where S3is the appropriate Ewald sum3*over all coupled transition dipole pairs in the adlayer. We will now take advantage of the fact that the geometric arrangement of the adlayer, and the corresponding Ewald sum, is not likely to change upon isotopic substitution to obtain
La
(13) where (A3H/A3D) is the ratio of the splitting produced by adlayers of C2H2and C2D2. Inserting the above values for !piH'and IpID1' into eq 13, we predict that (A3H/A3D) = 1.4 f 0.4. Since we observe (bH/bD) to be 2.0 f 0.1, just outside the estimated error limits of the predicted ratio, it is possible that higher order multipole moments may be required to successfully model the splitting. Since we have no values for the transition quadrupole or higher order moments, we must settle for this approximate quantitative analysis. Structure: Sublattice I. There are at least three energetically plausible adsorption sites for sublattice I: one which places the molecular axis perpendicular to the surface and two with the molecular axis nearly parallel to it.' Placing the acetylene molecule perpendicular to the surface directly above a chloride ion presents a favorable quadrupole-field gradient interaction and allows for the possibility of a weak hydrogen bond between an acetylenic hydrogen and the ion.' However, our polarization measurements strictly forbid a geometry with acetylene perpendicular to the surface. Another plausible adsorption site has the molecular axis of acetylene along the C1-C1 diagonal. Again this configuration contains a favorable quadrupoleelectric field gradient interaction. However, this adsorption site contains only a 2-fold axis of symmetry and eliminates the possibility of adsorbed molecules arranging themselves in T-shaped pairs in order to hydrogen bond. Since we know that both the van der Waals dimers and the low-temperature crystali2are made up of such pairs, and we know from the isotherms that significant intramolecular attraction exists, we consider this site unlikely. A third adsorption geometry has the center of the acetylene molecule, where the 7r electron density is high, directly over a sodium ion with the molecular axis bisecting chloride ions on either side (see Figure 5). This geometry provides favorable quadru-
I Cia Figure 6. Proposed bilayer structure for acetylene on NaCl( 100). (a) Sublattice I: the molecules are tilted 81 f 5 O from the surface normal. (b) The dark molecules are the half-filled second layer (sublattice 11) and are tilted 79 f 5' away from the surface normal. The a, b, c axes in a and b are used for symmetry analysis of the first and second layers, respectively, while the x, y, z axes are from Figure 1. The unit cell for each sublattice is encompassed by a dashed line.
pole-electric field gradient interaction and maximizes dispersion interactions.' It also contains a 4-fold axis of symmetry which allows adsorbates to form T-shaped, hydrogen-bonded pairs. Finally it allows relatively easy packing of one acetylene molecule per surface ion pair. For these reasons we consider this geometry a likely candidate. The low-temperature crystal structure contains hydrogen bonded sheets of molecules. All molecules within a layer are coplanar, and the unit cell is almost square with lattice constants a = 6.193 A and b = 6.005 A.12 It is possible to fit a layer of this structure with minimal overlap of the onto a NaCl lattice (a = 5.64 van der Waals radii if the molecules are slightly tilted, as our polarized measurements indicate, with respect to the surface plane (see Figure 6a). This tilt is indicated in Figures 5 and 6 by the shading of one hydrogen atom in each molecule. The tilt and rotation angles are characterized by a and @ of Figure lb, respectively . Sublattice 11. Sublattice I1 produces only one IR signal. The simplest explanation for this behavior is that there is only one
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5289
Structure of Acetylene on NaCl( 100) TABLE II: Cbaracter Table: Clh Isomorphic Plane Group E G i
4 4
AU
B U
1 1 1 1
1 -1 1 -1
1 1 -1 -1
1 -1
-1 (pa) 1 b b , Pc)
molecule per unit cell. Consider a half-filled second layer on top of sublattice I (see Figure 6b). This places the molecules in the second layer at either a near-T or nearly slipped parallel position with their first layer neighbors. The molecules in sublattice I1 also have their molecular axes along the C1-Cl diagonal of the surface, a previously discussed possible adsorption site. Trying to fill in the other half of this layer would result in unfavorable surface-adsorbate interactions. The bilayer structure in Figure 6b is energetically reasonable. Bilayers are not uncommon for adsorbates that exhibit strong intermolecular interactions such as hydrogen bonding. Thiel and Madey have done an extensive review of systems containing adsorbed water and reported that water forms bilayer structures on a number of metal surfaces even upon chemisorption where the surfaceadsorbate interactions are certainly greater than the lateral ones.39 Falsch, Stock, and Henzler have shown by ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED) that water also forms a bilayer on NaC1(100).40 Polanyi et al. have recently shown, in a polarized IR study, that HBr forms a stable bilayer structure on LiF( 100) in which the second layer appears before the first is ~ompleted.~’ In all of the above examples the adsorbates produce non-Langmuir isotherms that result from strong lateral interactions. Because acetylene on NaCl clearly exhibits these strong interactions between adsorbed molecules, a bilayer structure is physically reasonable. If sublattice I1 forms the second layer, then we might expect to see evidence for the first layer before growth of the second layer occurs. This would result in peaks in the doublet region appearing before the singlet. N o such behavior is seen. Why then would the first and second layers grow in simultaneously? Consider the following possible mechanism for the growth of acetylene on NaC1. When a small collection of molecules adsorb onto the substrate in the pattern portrayed in Figure 6a, the next most favorable site is not an empty surface site, but a position in the second layer pictured in Figure 6b. That is to say, the existence of a small island of sublattice I creates sites for sublattice I1 that contain not only favorable interactions with the surface but also with the neighboring molecules in the layer below. These second layer sites are then filled as quickly as they are created, leading to simultaneous growth of the first and second layers. The spectral complications seen for less than full coverage are probably due to islands of different size and orientation (there are four equivalent lattice directions of NaCl). At saturation the molecules are arranged in large, regular arrays of both sublattices yielding the three sharp peaks we observe. This proposed growth mechanism is thus consistent with all our results. Group Theory and Vibrational Symmetry Analysis. As shown by Berg and E ~ i n ga, type ~ of symmetry theory, using plane or slab groups, is a good way to qualitatively test a proposed monolayer structure against observed spectra. In the spirit of their development, we ignore the existence of the surface and consider the symmetry of two slabs consisting of either sublattice I or I1 of the bilayer. We use the 80 plane or slab group which describe the 80 unique patterns which can be produced by repeating a three-dimensional unit cell on the points of a two-dimensional lattice (see Table I from ref 4). These plane groups contain pure translational elements and nonsymmorphic elements: screw axes (C?) and glide planes (aB). When the translational operations are factored out of the plane groups, the resulting “factor groups” are isomorphic to point groups. This reduction occurs when the nonsymmorphic operations are interpreted as simple rotations and reflections. The proposed structure for sublattice I in Figure 6a contains C:), a glide four symmetry operations: identity ( E ) ,a screw axis ( plane (uag),and inversion centers (i). The a, b, c axes used for
symmetry analysis are coincident with the x, y, z axes from Figure 1. The screw axis and glide plane are shown in Figure 6a. A point of inversion is at the center of each molecule. These operations place the structure4 into plane group 23 which corresponds to a factor group C,. The character table for the C,, isomorphic plane group is given in Table 11. The out-of-phase collective mode is of a, symmetry and has its transition dipole moment along the a axis, or parallel to the surface. We see from geometrical considerations of Figure 6a that the out-of-phase mode for this structure will be parallel to the surface regardless of the angle 8. The in-phase mode possesses 6, symmetry. If3!, = 4 5 O and 135’ for neighboring molecules (a perfect T), the transition dipole moment for the in-phase mode lies at a 7 8 O angle away from the surface normal. We can determine the tilt angle for the two collective modes of sublattice I. By fitting Lorentzian line shapes to the two co_mp_onentsof the doublet in Figure 2 (see Table 11) and using (APIA,)for each peak, eq 8 gives cy = 88 f 5 O and cy = 75 f 5 O for the low- and high-frequency components, respectively. Within experimental error the low-frequency mode has its transition dipole parallel to the surface in agreement with the out-of-phase collective mode of the structure in Figure 6a. The tilt for the transition dipole of the high-frequency mode is within experimental error of that for the in-phase collective mode of this structure. The structure in Figure 6a is consistent with our experimental observations of sublattice I. The proposed structure for sublattice 11, shown in Figure 6b, has the following symmetry operations: identity (E), a 2-fold axis of rotation (Ca),a reflection plane (go), and inversion centers. The rotation axis and reflection plane are shown in Figure 6b, while the points of inversion are a t the center of each molecule. Sublattice I1 falls under plane group 12 which again corresponds to the C,, factor group (Table 11). Since there is only one molecule per unit cell, the only allowed collective mode has all of the molecules in sublattice I1 vibrating in phase. This mode possesses b, symmetry. Because the b, symmetry species allows for interaction with electric field vector components along both the a and c axes, a mode of this symmetry can have its transition dipole oblique to the surface as our measurements dictate for the collective mode produced by sublattice 11. Summary Acetylene can be adsorbed onto the (100) face of NaCl. Structural information, such as molecular tilt and orientation, can be extracted from polarized FTIR spectra. With reasonable models for bonding arrangements and group theoretical arguments, a bilayer structure has been proposed for the adsorbed phase. However, complimentary techniques, such as LEED and helium diffraction, which give lattice spacings for the adlayer are needed to obtain a unique structure.
Acknowledgment. We wish to thank Otto Berg, Rob Disselkamp, and William Schaich for their help and advice and the National Science Foundation for financial support. References and Notes (1) Schinzer, W. D. Ph.D. Thesis, Indiana University, 1981. (2) Berg, 0.; Quattrocci, L.; Dunn, S. K.; Ewing, G. E. J. Elect. Spec. Rel. Phenom. 1990, 54155,981. (3) Richardson, H. H.; Ewing, G. E. J . Phys. Chem. 1987, 91, 5833. Ewing, G. E. Int. Rev.Phys. Chem. 1991, IO, 391. (4) Berg, 0.;Ewing, G. E. Surf. Sci. 1989, 220, 207. ( 5 ) Heidberg, J.; Kampshoff, E.; Suhren, M. J. Chem. Phys. 1991, 95, 9408. (6) Heidberg, J.; Kampshoff, E.; Schbnekiis, 0.;Stein, H.; Weiss,
H. Eer.
Bunsenges. Phys. Chem. 1990,94, 118. Heidberg, J.; Kampshoff, E.; Kuhnemuth, R.; Schbnekis, E. Surf. Sci. 1991, 251/252, 314. (7) Hardy, J. P.; Ewing, G. E.; Stables,R.; Simpson, C. J. S. M. Surf, Sci. 1985, 159, L474. (8) Fraser, G. T.; Suenran, R. D.; Lovas, F. J.; Pine, A. S.;Hongen, J. T.; Lafferty, W. J.; Muenter, J. S.J . Chem. Phys. 1988, 89, 6028. (9) Prichard, D. G.; Nandi, R. N.; Muenter, J. S.J . Chem. Phys. 1988, 89, 115. (IO) Prichard, D.; Muenter, J. S.;Howard, B. J. Chem. Phys. Let?. 1987, 135, 9.
J. Phys. Chem. 1992, 96, 5290-5295 (11) Bone, R. G. A.; Murray, C. W.; Amos, R. D.; Handy, N. C. Chem. Phys. Lett. 1989, 161 (2), 166. (12) Koski, H. K.; Sandor, E. Acta Crysrallogr. 1975, 831, 350. (13) Ito, M.; Yokoyama, T.; Suzuki, M. Spectrochim. Acra 1970, Z6A, 694
1 _ _ .
(14) Fisher, G.; Miller, R. E.; Vohralik, P. F.; Watts, R. 0.J . Chem. Phys. 1985,83, 147 1. (15) Miller, R. E.; Vohralik, P. F.; Watts, R. 0.J . Chem. Phys. 1984.80, 5453. (16) Bryant, G. W.; Eggers, D. F.; Watts, R. 0.J . Chem. Soc., Faraday Trans. 2 1988,84 (9), 1443. (17) Bottger, G. L.; Eggers, D. F., Jr. J . Chem. Phys. 1964, 40, 2010. (18) Marchi, M.; Righini, R. Chem. Phys. 1985, 94, 465. (19) Anderson, A.; Smith, W. H. J . Chem. Phys. 1966, 44,4216. (20) van N e , G. J. H.; Frt van Bolhuis Acra Crysfallogr.1979,835, 2580. (21) Aoki, K.;Kakudate, Y.; Usuba, S.;Yoshida, M.; Tanaka, K.; Fujiwara, S.J . Chem. Phys. 1988, 88, 4565. (22) Khanna, R. K.; Ospina, M. J.; Zhao, G. Icarus 1988, 73, 527. (23) Chang, H. C.; Richardson, H. H.; Noda, C.; Ewing, G. E. J . Chem. Phys. 1988.89, 7561. (24) Richardson, H. H.; Chang, H. C.; Ewing, G. E. Surf. Sci. 1989, 216, 93. (25) Gaudin, A.; Hagemann, R. J . Chim. Phys. 1967, 64, 1209. (26) Burrell and Robertson, Bureau of Mines, Tech. Papers 142.
(27) Hill, T. An Introduction Io Stafisfical Thermodynamics; AddisonWesley: Reading, MA, 1960. (28) Fowler, R. H.; Guggenheim, E. A. Sratisrical Thermodynamics; Cambridge University Press: London, 1960. (29) Richardson, H. H.; Baumann, C.; Ewing, G. E. Surf. Sei. 1987,185, 15. (30) Lafferty, W. J.; Thubault, R. J. J. Mol. Spect. 1964, 14, 79. (31) Kim, K.; King, W. T. J . Mol. Sfruct. 1979, 57, 201. (32) Sherwood, P. M. A. Vibrarional Spectroscopy of Solids; Cambridge University Press: London, 1972. (33) Palik, E. D. Ed., Handbook of Oprical Conrtanrs of Solids; Academic Press: Orlando. FL. 1985. (34) Benedeh, G:;Brusdeylus, G.; Doak, R. B.; Skofronick, J. G.; Toennis, J. P. Phys. Reu. 1983, 828, 2104. (35) Disselkamp, R.; Chang, H.-C.; Ewing, G. E. Surf. Sei. 1990,240, 193. (36) Wilson. E. B.. Jr.; Decius. J. C.: Cross. P. C. Molecular Vibrurionr: McGraw-Hill: New York, 1955. (37) Hexter, R. M. J . Chem. Phys. 1960, 33, 1833. (38) Kittel, Introducrion fo Solid Stare Physics, 6th ed.; Wiley: New York, 1986; Appendix B. (39) Thiel, P. A.; Madey, T. E. SurJ Sei. Rep. 1987, 7 , 211. (40) FBlsch, S.;Stock, A.; Henzler, M. Sur$ Sei. 1992, 264, 65. (41) Blass, P. M.; Jackson, R. C.; Polanyi, J. C.; Weiss, H. J. Chem. Phys. 1991, 94, 7003.
Infrared Spectra and Structure of Perfluoro-n-alkanes Isolated in n-Alkane Matrices Prepared by Vapor Deposltion Han-Cook Cho, H. L. Strauss, and R. C. Snyder* Department of Chemistry, University of California, Berkeley, California 94720 (Received: December 4, 1991; In Final Form: March 3, 1992)
Perfluoro-n-alkane chains (n-C,F2,,.+I,n = 14, 16, 20, and 24) diluted in n-alkane matrices have been prepared in the form of amorphous films by means of low-temperature vapor deposition. Annealing these films leads to conformational ordering of both kinds of chains. Because of reduced intermolecular interaction between the perfluoro-n-alkane chains, narrow infrared bands result. This has allowed the observation of many intense bands in the 1100-1300-~m-~region that have not previously been resolvable. The intensities and frequencies of C-F stretching bands found in the El zone-center region of the v3 band vary with the chain length. These bands have been assigned, and their unusual intensity distribution in the zone-center region has been accounted for on the basis of a simple coupled oscillator model. Our study shows that the isolated perfluoro-n-alkane chains are highly ordered and persist as helices.
Introduction Spectroscopic information on fluorocarbons is of great importance from a theoretical as well as practical view point. Infrared spectroscopy has played a key role in analyzing the structure and dynamic behavior of fluorocarbons.'-' For example, the band at 778 cm-I is widely used for the determination of the crystallinity of poly(tetrafluoroethy1ene) (PTFE).Z Much of the complexity of the fluorocarbon spectra is attributable to the well-known helix structure of the chains.s Bunn and Howells first reported that there were two helical structures of PTFE, one (1517) above the room-temperature solidsolid transition and one (13/6) below.8 It is generally accepted that the helix structure is a result of the large van der Waals' radius of the fluorine atom. The helix that exists above room temperature is looser because some disorder, which is associated with chain rotation or translation, is introduced at the transition point. Fluorocarbon chains tend to be more extended than n-alkanes, especially in the melt:JO due to the higher value of E," (the energy difference between the gauche and the trans conformations). A I3C N M R study has shown that internal reorientation motion about the C-C bond of the fluorocarbon chain in the liquid state is much reduced relative to its alkane counterpart." Bunn and Howells reported that the perfluoro-n-alkane, C16Fj4,has almost the same helix structure as PTFE.8 Since the infrared and Raman spectra of PTFE and perfluoro-n-alkanes show close similarity, the perfluoro-n-alkanes have been used as model compounds in order to understand the spectroscopy of PTFE.'Z-15
Despite many studies, the prominent bands in the C-F stretching region (1 100-1 300 cm-I) of fluoroalkanes are not well understood, mainly because they are broad and very intense. Since Liang and Krimm first attempted to assign the bands from dichroic measurement and normal-mode calculations, assuming a planar zigzag structure,' there have been many other infrared and Raman s t ~ d i e s . ~ ~ ~ However, - ~ J ~ J ~the analyses of the broad absorption profile that is observed for the perfluorocarbons have been controversial and often misleading.16 As a result, the assignments of the bands, especially in the v2 band, antisymmetric CF2 stretching, region (1200-1260 cm-I) have often been revised and are not yet reconciled with the results of the dichroic measurements.'6J8~19 Hsu et al. succeeded in forming amorphous films of perfluoro-n-alkanes by means of vapor deposition.' At low temperature (7.5-50 K), the spectrum (800-1 300 cm-I) of such a film resembles that of the liquid perfluorocarbon. As the temperature of the film is increased, the bands arising from the conformationally disordered perfluorocarbon chains slowly disappear and the v1 and the u3 progression bands that characterize the conformationally ordered chain become visible. However, the zonecenter regions of the intense and broad vZ and v3 bands remain ill resolved. In this paper, we report an infrared study on perfluoro-n-alkanes isolated in n-alkane matrices. Vapor of a perfluoro-n-alkane and that of an n-alkane were deposited together in vacuum onto a cold window (-7.5 K), and an amorphous film was formed. The film was subsequently annealed in steps. The infrared spectra measured
0022-365419212096-5290$03.00/0 0 1992 American Chemical Society
