J . Phys. Chem. 1989, 93, 6025-6028
6025
Selective Trapping of the Complexes [OC***HF] and [ C O H F ] by Photodissociation of Matrix- Isolated Formyl Fluoride Gabriele Schatte, Helge Willner,* Detlev Hoge,' Institut fur Anorganische Chemie und Institut f u r Physikalische Chemie der Universitat Hannover, 0-3000 Hannover, FRG
Erich Knozinger, and Otto Schremst Institut fur Physikalische Chemie der Universitat Siegen, 0-3900 Siegen, FRG (Received: December 13, 1988; In Final Form: February 22, 1989)
The molecular complexes [OC.-HF(DF)] (1) and [CO--HF(DF)] (2) have been selectively formed in high yields by UV photofragmentation of matrix-isolated formyl fluoride (HC(O)F, DC(0)F). The less stable form 2, where the oxygen atom of CO acts as proton acceptor, is produced at considerably lower concentrations compared to the more stable form 1. It has been possible to analyze the whole set of normal modes of both complexes by means of FTIR spectroscopy. The hydrogen bond of H F to the carbon or oxygen atom of CO was verified by comparison of measured 13Cand l 8 0 isotopic shifts of v(C0) with calculated shifts. Annealing experiments showed that metastable CO/HF species increase the concentration of both 1 and 2 and, more importantly, the ratio of concentration 1:2 was changed. The enthalpy difference between the isomers at 20 K was estimated from relative intensity measurements with neglect of the A S term: [OC--HF] + [CO-HF]; AHo = +0.3 kJ/mol. All frequency shifts of CO induced by the complex formation can be understood on the basis of a simple model for the hydrogen bond.
Introduction Hydrogen fluoride (HF) is an excellent proton donor molecule for the study of hydrogen bonding with diverse proton acceptor molecules.' Diatomic molecules that are able to form isomeric complexes with H F are of particular interest for both experimentalists and theoreticians. A highly interesting example is the C O / H F complex, which-in principle-is able to form three different isomeric species: COC*-.HFl 1
CCO*.*HFI 2
III***HF
0
3
It can be compared with the well-known isoelectronic N 2 / H F and C 2 H 2 / H F complexes.' Various a b initio calculations provided structural, vibrational spectroscopic and thermodynamic data for complexes 1 and 2.2-5 According to the calculations complex 1 should be more stable by 1-3 kJ mol-' in comparison to complex 2. It was shown that the most favorable pathway for the interconversion of complex 2 into complex 1 is the one that allows favorable dipole-quadrupole interactions.2 In another calculation it was found that the relative stability of the two complexes is reversed at temperatures above 400 K.3 The first experimental evidence for species 1 was obtained by microwave spectroscopy.6 However, a second isomer has not been detected by this technique. The intermolecular distance between the two molecules in the linear complex 1 was calculated on the basis of the microwave data as 125 pm. From the centrifugal distortion constant the frequency of the intermolecular stretching vibration u(0C-HF) was derived as 125 cm-l. Complex 1 has also been identified in the gas phase by IR spectroscopy.' Further fundamental modes of this complex became available from matrix experiments.8 Their frequencies are v(HF) = 3789.3 cm-', v ( C 0 ) = 2162.4 cm-l, and G(OC-HF) = 389.5 cm-'. It was very recently that in a further matrix study a band at 3907 cm-' was detected as the first experimental evidence for the presence of the second isomer (complex 2).9 So far, in all matrix studies the C O / H F complexes were produced by codeposition of CO and H F with the matrix gas. This 'Present address: Fritz-Haber-Institut, Faradayweg 13, D-1000 Berlin, FRG. *Present address: Alfred-Wegener-Institut, Sektion Chemie, Columbusstrasse, D-2850 Bremerhaven, FRG.
0022-3654/89/2093-6025$01.50/0
deposition technique gives rise to complex spectra because of the simultaneous presence of various aggregated species. Furthermore, the complex is formed only in relatively low concentrations. Consequently, the IR and far-IR detection and assignment of the vibrational fundamentals of the complexes under investigation is more difficult and remains incomplete. Thus, we have produced CO/HF(DF) complexes in cryogenic matrices selectively by photodissociation of matrix-isolated H C ( 0 ) F (DC(0)F). This method enables us to achieve considerably higher concentrations of the complex species in the matrix and under appropriate conditions prevents the formation of complexes other than 1:l types.
Experimental Section Synthesis of Formyl Fluoride. H C ( 0 ) F and D C ( 0 ) F were prepared according to the literature methodlo KHF2 HCOOH + P h C ( 0 ) C l H C ( 0 ) F K F HCI + PhCOOH (1)
+
+
+
The gaseous products from the reaction of 6 g of KHF2, 3.5 g of HCOOH (DCOOD), and 10 g of benzoyl chloride were trapped at -183 O C and purified by trap-to-trap fractionation under vacuum in a series of traps held at -80, -1 30, and -1 96 "C. The pure formyl fluoride in the trap at -130 O C was then transferred into a glass ampule and stored at -196 "C. Formyl fluoride has its boiling point at -28 O C . It is sensitive to hydrolysis and decomposes at ambient temperature into H F and CO. Metal fluorides strongly catalyze this reaction. The (1) Andrews, L. J . Phys. Chem. 1984.88, 2940.
(2) Benzel, M. A.; Dykstra, C. E. Chem. Phys. 1983, 80, 273. (3) Slanina, Z. Thermochim. Acta 1986, 102, 287. (4) Benzel, M. A.; Dykstra, C. E. J . Chem. Phys. 1983,78,4052. Benzel, M. A.; Dykstra, C. E. J. Chem. Phys. 1984,80, 3510. Hincliffe, A. Adu. Mol. Relax. Interact. Processes 1981, 21, 151. (5) Curtiss, L. A.; Pochatko, D. J.; Reed, A. E.; Weinhold, F. J . Chem. Phys. 1985, 82, 2679. Botschwina, P., private communication. (6) Legon, A. C.; Soper, P. D.; Keenan, M. R.; Minton, T. K.; Balk, T. J.; Flygare, W. H. J . Chem. Phys. 1980, 73, 583. Legon, A. C.; Soper, P. D.; Flygare, W. H.J . Chem. Phys. 1981, 74, 4944. (7),Kyro, E. K.; Shoja-Chaghervand, P.; McMillan, K.; Eliades, M.; Danzeiser, D.; Bevan, J. W. J . Chem. Phys. 1983, 79, 78. Fraser, G. T.; Pine, A. S. J . Chem. Phys. 1988, 88, 4147. (8) Andrews, L.; Arlinghaus, R. T.; Johnson, G. L. J . Chem. Phys. 1983, 78, 6347. (9) Andrews, L.; Hunt, R. D. J . Phys. Chem. 1988, 92, 81. (10) Olah, G. A.; Kuhn, S . J. J . A m . Chem. SOC.1960, 82, 2380.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989
6026 0.8
1
r
Schatte et al. TABLE I: Measured Wavenumbers (cm-') for HC(0)F and JX(0)F
HC(0)F 4.
3650 w 2982 w 2115 w 1835 vs
0.L
-0.e
1717vw 1344 vw 1066 vs
+
lI "1
4 4 LOO0
3000
2000
t
l ~
I
662 m
3637.6 2996.25 2097.3
DC(0)F gas phase Ar matrix 3576vw 2260 s 2139 w 2027 vw 1929 vw 1794 vs
1829.5 1791.1 1788.1 1712.35 1701 m 1343.0 967 s 1055.9 1074 vs 1031.9 (1038.2?) 857 vw 664.4 658 m
2271.6 2123.8 2020.4 1928.5 1790.7 1758.6 1703.6 966.9 1067.0 1044.8 867.8 659.1
assignment
202 u,
2uq ul
+ u4
2u3 u2 u2
u(C0)
u2
'80
I3C
u 4 + u5
u(CH) 4CF)
~3
u4 u4
"C
6,,(CH) 6(OCF)
~g
us
T
Y'
1OW
. cm.'
IOC HFI
v2 IOC HFI
I 1
50
Figure 1. Difference IR spectrum of HC(0)F and its photolysis products: HC(0)F:Ar = 1:500 (1.8 mmol of matrix); resolution 1 cm-I.
UV spectrum of the gaseous sample exhibits a broad absorption band around 210 nm with a characteristic vibrational fine structure. Instrumentation. Formyl fluoride and argon were mixed in a high-vacuum system (Duran glass) at molar ratios of 1:400 to 1:2000. The mixture was transferred to a quartz glass capillary to the matrix-isolation equipment, and the gas flow was controlled by a needle valve. The sample support was a highly reflecting nickel-plated copper block that may be cooled down to a minimum temperature of 4 K by the aid of a liquid helium continuous-flow cryostat (Cryovac, Troisdorf, FRG). A sample of 0.5-2.0 mmol of the matrix was deposited at a temperature of 10 K and a rate of about 4 mmol/h. The C O / H F complexes have been obtained selectively, in situ, by photolyzing the matrix containing isolated formyl fluoride:
-0.05-
-0.05 3900
3850
3800
cm '
2180
2160
2lL0
cm .
Figure 2. IR spectrum of CO/HF complexes in Ar matrix from Figure 1 in expanded scale at 12 K (a), after annealing to 18 K (b), and after annealing to 24 K (c). *: metastable species.
Results and Discussion The irradiation of matrix-isolated formyl fluoride with UV light ( A < 250 nm) induces an almost quantitative decomposition into C O and HF. The photofragments, which are born vibrationally
and rotationally excited, are trapped in the matrix cage formed by the parent molecule. It has been shown that photolysis of formyl fluoride leads to H F infrared laser emission.12 In our experiments the excess energy of the photofragments can be released to the phonon bath provided by the matrix. Thus, only very small fractions of H F and C O escape from the cage by diffusion and give rise to a weak R(0)band of H F at 3962 cm-l and a slight increase of both intensity and width of the CO band at 2138 cm-I. In the IR spectra of matrix-isolated H C ( 0 ) F no impurities could be detected with the exception of traces of CO, originating from the decomposition of formyl fluoride in the needle valve. In matrices containing high concentrations of H C ( 0 ) F ( M I A = 500), the IR spectra exhibit unambiguous evidence of HC(0)F-dimer formation. Characteristic dimer bands of formyl fluoride, marked in Figure 1 by D, were found at 1818.3 and 672.7 cm-I for the undeuterated compound and at 2285.6, 2146.2, 1782.6, 1718.8, 1075, 959.6, and 667.5 cm-' for the deuterated compound. Consequently, the deposition conditions had to be optimized such that the formation of dimers and higher aggregates could be excluded. The IR data obtained by formyl fluoride in the course of this study are presented in Table I, which is more comprehensive than the literature data existing so far.13J4 The process of photolysis is best illustrated by spectral substraction of the data related to the parent molecule from those related to the fragments. Figure 1 shows the respective difference
( 1 1 ) Knozinger, E.; Schrems, 0. Vibrational Spectra and Structure; Durig, J . R., Ed.; Elsevier: Amsterdam, 1987; Vol. 16, p 158. Schrems, 0.; Huth, M.; Kollhoff, H.; Wittenbeck, R.; Knozinger, E. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 1261.
(12) Klimek, D. E.; Berry, M. J. Chem. Phys. Lett. 1973, 20, 141. (13) Morgan, H. W.; Staats, P. A.; Goldstein, J. H. J . Chem. Phys. 1956, 25, 337. (14) Mizuno, M.; Saeki, S. Specrrochim. Acta 1978, 3 4 4 407.
(2)
The photoproducts H F and C O are trapped in matrix cages preformed by the parent molecule H C ( 0 ) F . The molecular photoelimination (eq 2) predominates near the S1 threshold (n T*). The photolysis experiments have been carried out by using a medium-pressure mercury arc lamp T Q 150 (Heraeus, Hanau, FRG), 1 : 1 quartz optics, and a water filter of 2-cm thickness. After an irradiation period of 15 min the yield of C O / H F was about 90%. The IR and far-IR spectra were recorded with a FTIR spectrometer IFS 113v (Bruker, Karlsruhe, FRG) at a resolution of 1 cm-I in the reflection mode. A reflectance attachment was mounted in the sample compartment to bounce the I R beam of the interferometer to the cold remote mirror holding the matrix and return it then to the usual optical path." A liquid nitrogen cooled InSb detector, a liquid helium cooled GeCu detector, and a liquid helium cooled Ge bolometer were applied for recording the infrared spectra in the intervals 4000-2000, 2000-350, and 350-20 cm-I, respectively.
-
Selective Trapping of [OC--HF] and [CO-HF]
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989 6027
TABLE 11: Calculated and Measured Wavenumbers (em-') of HF, CO, IOC-HFI, and ICO-HF1 Isolated in Ar Matrix HF
CO
3920' 41 19 2138.3 2307 2091.3 2087.4 40
[OC*-HF]
[OC-DF]
3789.9 vs 3789 4089 2162.1 s 2162.4 2350 2114.4 21 10.9 389.3 s 389.5 436 103 vw 125 151 45 ww 115
2781.5 278 1 2163.4 2163.4
[CO.-HF]
[CO-.DF]
3908.0 3907 41 IO 2129.3
2867.1 2866
387 107 (sh)6
105 127
ref this work 8, 9 2' this work
4HF) WF) 4CO) 4CO) 4CO) u(13c0) u(C'80) (in-phase libration) (in-phase libration) (in-phase libration) v(CO/HF) u(CO/HF) u(CO/HF) (counter-phase libration) (counter-phase libration)
2128.7
2279 2082.4 2078.4 346 (br)b
2115.7 2112.2 304.7 305.5
assignment
208 1.8 2077.9 255 (br)b 109 (sh)6
151 151
8
2' this work this work this work 8
2c this work
5 2' this work 2c
Q induced. Absorption uncertain (see text). cCalculated values. v3
C
0.03
0.5
u 4 K
s1m
. I
0.25
II
ICO.'.HF I
A
;
0 140
420
400
380
360
340
320
WRVENUHEERS C H - 1
380
280
o
\/
, 120
I 100 80 60 WRYENUMBERS C M - I
40
20
Figure 3. Far-IR spectra of CO/HF complexes in Ar matrix from Figure 1 in expanded scale at 12 K (a) and after annealing (b); CO/DF after annealing (c,. *: metastablk species. '
spectrum in the range between 4000 and 50 cm-I. In Figure 2 the spectral intervals containing the H F and the C O stretching vibrations are presented after appropriate scale expansion. There is clear evidence for at least four different species, which may be discriminated by annealing experiments. On warming the matrix to 24 K and recooling it to 12 K all bands marked with an asterisk (*) lose relative intensity, whereas the opposite is true for the bands assigned to the two complexes [OC--HF] (1) and [CO-HF] (2). The bands marked with an asterisk (*) belong to metastable C O / H F species-perhaps 1 with argon vacancies nearby, and/or the n-complex 3. The bands (*) are located at 3796.0, 3775.0, 2168.0, and 421.Ocm-I for C O / H F and at 2784.5, 2771.5, 2169.5, and 325.5 cm-I for CO/DF. By annealing the matrix, these metastable species increase the concentration of both 1 and 2 and, more importantly, the ratio of concentration 1:2 is changed. The different local minima contribute to the relatively complicated spectra in Figure 2. Annealing clearly induces a rearrangement such that the energetically most favorable structures prevail. The respective band positions coincide perfectly well with those obtained in the course of codeposition experiment^.^^^ Only these data are incorporated in Table I1 and in the assignment indicated in Figure 2. Within the periods of annealing that we have supplied, it was not possible to eliminate the unassigned bands (*) completely. In Figure 3 the far-IR spectra of the complexes obtained by photodissociation of matrix-isolated formyl fluoride are shown in greater detail. They cover the frequency range of the intermolecular modes of the complexes (450-20 cm-I). Our own d e p o s i t i o n experiments ( H F C 0 : A r = 1:1:500) led to band positions of the two C O / H F complexes 1 and 2 at 3908.0,
3789.4, and 2162.2 cm-', which are in good agreement with those reported in ref 9. In addition, we found a band at 2130 cm-' buried under the low-frequency wing of the CO band and not recognized in ref 9. Assignment ofthe Bands. The experiments presented here and quantum chemical calculations*~5 clearly show-in agreement with ref 9-that the intermolecular potential energy surface of the C O / H F complex has two global minima of only slightly different energy. The corresponding structures 1 and 2 have C,, symmetry. The irreducible representation based on the respective normal coordinates is rvib= 3 2 + 2 n (3) The symmetry class 2 comprises the intramolecular stretching vibrations u1 = u(HF) and v2 = u(C0) as well as the intermolecular H bond stretching mode u j . The vibrations in the symmetry class II,on the other hand, are 2-fold degenerate and exhibit bending character. v4 is a mixed translation/in-phase libration of HF and C O (Figure 4). Here the proton acceptor atom (C or 0)moves in the opposite direction compared with the proton itself. The restoring force is, therefore, stronger than that for the other bending mode, the pure counter-phase libration u5 (Figure 4). The assignment of the bands presented in Figures 1-3 to the species 1 and 2 is based on (a) their concentration ratio which should influence the intensity ratio (Z[ui( l)]/Z[vl(2)]) qualitatively in the same way for all normal vibrations (i) and (b) the calculated a b initio frequencies. Obviously the concentration of 2 is comparatively low. This causes difficulties in locating the respective
6028
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989
coordinate R ) and the CO bond (normal coordinate r ) . They were obtained from the anharmonicity-corrected frequencies of the different isotopomers and provide relevant structural information on the C O / H F complexes:
"L
OC IC01
oc
HF
equilibrium
(COI
HF
non-equilibrium
"5
OC K O )
-.
HF
oc
cquiiibri
(COI
HF
/- " \
....-
non- e q u l l , brium
dm
Figure 4. Schematic representation of the two intermolecular modes u4
and
u5
of the symmetry class II in a linear heterodimer.
TABLE 111: Characteristic Shifts Aw2 (Eq 4) between Y ~ ( ' ~ C and ) v2(**O)in the Complexes of Carbon Monoxide and Hydrogen Fluoride
complex
Awl c a l d
Au2 ok3d
[ OC*-HF]
3.5 3.5
3.5 3.5
4.0 4.0
4.0
[OC.-DF] [ CO-H F] [CO-DF]
3.9
modes. The v4 band appears to be weak and broad. Its assignment is, therefore, uncertain. The same is true for v3, which may be related to a shoulder on the high-frequency side of a very weak band at 103 cm-' assigned to u3 of structure 1. An extremely weak band at 45 cm-I could originate from the counter-phase librational mode in structure 1. Its counterpart in structure 2 has not yet been identified and is expected below 45 cm-l. In complex 1 the frequency of v(C0) is increased and in complex 2 decreased compared to the value for the matrix-isolated C O molecule. These shifts can unambiguously be related to the MO scheme of the C O molecule as a consequence of the complex formation. The energy of the proton acceptor atomic orbital (C in 1 and 0 in 2) is lowered. In complex 1 (C is proton acceptor) this necessarily implies that the energy levels of the carbon and the oxygen atomic orbitals in C O become more similar-a fact that clearly favors bond strengthening with an inherent increase of the frequency of v(C0). If, in turn, the complex formation involves the oxygen as a proton acceptor, the gap between the energy levels of the carbon and the oxygen atomic orbitals in CO is increased. The C O bond is, therefore, weakened and the frequency of v(C0) is reduced. Owing to the large anharmonicity, the hydrogen bond becomes more stable on deuteration of the complexes. This is in agreement with the observation that deuteration causes an increase of the frequency of v(C0) in 1 and a corresponding decrease in 2 (Table 11). Model Calculations. The structure of a linear XYZ molecule can be related to isotopic shifts of vibrational frequencies.15 In the case of C O / H F and C O / D F complexes, Av2 values may be defined according to Av2 = v2(13C)- v2(l8O)
(4)
which supports the v(C0) assignment given above in terms of the isomer structures 1 and 2. They are accessible in two independent ways: (a) by a normal-coordinate analysis based on Wilson's GF matrix methodI6 and (b) by FTIR spectroscopy (see isotopic substitution data in Table 11). Table I11 shows that the assignment presented in Figures 2 and 3 is correct since the calculated and respectively, the experimentally observed data, Avlalcd and Avz are in good agreement. Other important results of the model calculations are the interaction force constantsfRr between the hydrogen bond (normal (15) Linton, H. R.; (16) Wilson, E. B.;
Schatte et al.
Nixon, E. R. J . Chem. Phys. 1958, 58, 990. Decius, J. C.; Cross, P. C. Molecular Vibrations; McGraw-Hill: New York, 1964.
[OC-*HF]:
fRr
= -(0.06 f 0.01) X IO2 N / m
(5)
[CO-HF]:
fR,
= +(0.03 h 0.01) X lo2 N / m
(6)
The negative sign implies that an increase of the H bond length induces a weakening of the C O bond. Thus the signs of the interaction force constantsf,, fully confirm the theoretical bond concept developed above. Enthalpy of [OC-HF] and [CO-.Hfl. As shown in ref 9, the concentration ratio of 1 and 2 obtained in codeposition experiments strongly depends on the deposition conditions. The freezing process is so fast that an efficient relaxation into the thermodynamic equilibrium is hardly possible. The less stable structure 2 remains overpopulated even after annealing. In our warm-up experiments the relative intensity of the bands attributed to the less stable species 2 increases more than those which relate to the species of higher stability 1 (Figures 2 and 3). Obviously, the thermodynamic equilibrium is approached from the opposite side in comparison with the codeposition experiment^.^ The final state attained does not depend on the experimental conditions and provides a concentration ratio (1:2) of 7 :1 at 20 K. By neglecting the entropy term, we can estimate the reaction enthalpy for the system: [OC-HF] F= [CO-HF] AHo = +0.3 kJ/mol ( 7 ) From the few IR spectra of the D C ( 0 ) F photodissociation products it was not possible to evaluate reliable intensity ratios 1:2 before and after annealing. The higher stability of 1 clearly correlates with the charge distribution of carbon monoxide which is described as a dipole exhibiting the negative charge on the C atom and the positive charge on the 0 atom.*'
Conclusion In this FTIR study the complete set of normal modes of [OC-HF] was recorded and assigned for the first time. This was made possible by selective generation of the C O / H F complexes in a solid Ar matrix. Clear experimental evidence for a second less stable species [CO-HF] has been found in agreement with ref 9. The vibrational frequencies presented here for both complexes are in good agreement with those calculated in ref 5. Considerable deviations are observed for the low-frequency modes. All frequency shifts induced by the complex formation may be understood on the basis of a simple model for the hydrogen bond. The small enthalpy difference estimated for [OC-.HF] and [CO-HF] should allow microwave detection of both species. In fact, only one complex has been observed so far.6 Of course, the stability of the less stable complex could be favored by the matrix cage, such that its population is considerably higher in Ar than in the gas phase. Furthermore, it is most likely that the rotational levels of the less stable complex [CO-.HF] are split into various sublevels owing to an extremely low lying frequency of the librational mode v5. This would necessarily imply a reduction of detection sensitivity. According to ref 1 the stability of H F complexes directly correlates with the frequency shift v(HF) induced by the H bond. The two C O / H F complexes do not fit in this correlation. Considering the large difference (1 18.1 cm-l) of the respective v(HF) values, AHo (eq 7) is by far too small (+0.3 kJ/mol).
Acknowledgment. We thank the Deutschen Forschungsgemeinschaft and the Verband der Chemischen Industrie e.V.Fonds der Chemie for financial support. (17)
Rosenblum, B.; Nethercot, A. H.; Townes, C. H. Phys. Rev. 1958,
109, 400.
