J. Phys. Chem. 1994, 98, 8310-8314
8310
Infrared Absorption Spectroscopy of the OCBr2 van der Waals Complex? Yaomin L i d and Robert A. Beaudet' Chemistry Department, University of Southern California, Los Angeles, California 90089-0482 Received: March 9, 1994; In Final Form: May 5, 1994"
High-resolution rovibrational absorption spectra of the weakly bonded OCBr2 complex have been observed in the region of the CEO stretching mode in the 2143-cm-I region. Four progressions originating from different bromine isotopic species were recorded. The spectra, which are consistent with a linear complex, were well fitted by a semirigid rotor Hamiltonian. The top five parameters in Table 2 were determined. The distances from the Br2 bond center to the CO center of mass are 4.884 and 4.893 A for the ground and excited states, respectively. Though the orientation of the CO has not been determined experimentally, we presume that it is similar to OCC4, namely, the halogen is bonded to the carbon atom.
Introduction During the past few years, spectroscopic studies of over 20 van der Waals complexes containing C O have been studied.1-20 The CO complexes with H X (X = F, C1, Br, and I) have been the most extensively studied systems. Fraser and Pinelo observed the hydrogen-bonded OCHF complex by exciting the HF vibrational stretching mode and determined that the OCHF complex is linear with the band origin red shifted from the HF monomer by 1 17.4cm-1. Wang and Bevan" obtained the infrared absorption spectrum of OCHF by probing the complex in the vicinity of the v = 1 u = 0 C W stretch. They found that the band origin of the CEO stretch is blue shifted by 24.43 cm-l from the C = O monomer. TheOCHC112,OCHBr,l3, and OCHI14 complexes were observed by exciting the I R absorption of the v = 1 v = 0 vibrational transition of the C a stretching mode. These complexes were found also to be linear with shifts in the band origins of 12.23, 9.23, and 5.28 cm-1, respectively. The blue shifts in the OCHX family were interpreted to be caused by the strengthening of the C m bond upon the formation of a van der Waals bond. The hypochromic shift of the band origin (AVO)for the OC(v'= 1)-HX OC(v'=O)-HX system decreases with halogen mass: OCHF > OCHCl > OCHBr > OCHI. The RgCO (Rg = rare gases) system also has attracted much attention. The spectra of the ArCO complex were characterized both in the free jet expansion in this laboratory and in a long-path low-temperature static gascell by McKellar et al.ls Subsequently, the pure rotational spectrum was measured by FT microwave spectroscopy.16 The rotational structure corresponds to an approximately T-shaped effective geometry with a van der Waals bond length (RcM)of 3.85 A and a band origin slightly red shifted by 0.44 cm-1 from the CO monomer. The ArCO complex appears to be a nonrigid species, with large amplitude bending. Recently, McKellar et al. also recorded the rovibrational spectra of the NeCO and HeCO complexed7J*in the long-path low-temperature static gas cell. Preliminary results showed that NeCO is T shaped with a band origin red shifted by 0.075 cm-I from the CO monomer. Binding between rare gases and CO appears to be weak and very flexible. Recently, we have investigated the infrared spectrum of the OCC1219complex by exciting the CEO stretching mode with a diode laser source. The observations indicated that OCCl:! is linear with the band origin blue shifted by 6.271 cm-I relative to
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~
~~
Research supported by the U S . Army Research Office Center for the Study of Fast Transient Processes and by the Department of Energy under Grant DE-FG03-89ER4053 (R.A.B.). t Present address: National Tsing Hua University, No. 59 East Compound, Tsing Chu City, Tsing Chu County, Taiwan. Abstract published in Advance ACS Abstracts, July 15, 1994. t
@
0022-365419412098-83 10$04.50/0
uncomplexed CO. The distance between the CO and Clz centers of mass was found to be approximately 4.78 A. Its distortion constant, 4.37 X 1 W cm-I, was smaller than expected for weaklybonded complexes with similar rotational constants, suggesting that this complex must be relatively tightly bound and inflexible. This strong van der Waals bonding was rationalized by invoking electron donation from the highest occupied molecular orbital (HOMO) in C O to the lowest unoccupied molecular orbital (LUMO) of Clz. However, the orientation of C O could not be determined without isotopic substitution on the CO subunit. Ab initio structure and vibrational frequency calculationsZoindicate that a weak u bond can form in the OCClz isomer, while no such bonding occurs in the COCl:! isomer. The calculation predicted that the ground vibrational state in OCCl2 is more stable than that in COC12 by approximately 165 cm-1. More recently, Jager et a1.21 observed the rotational spectrum of OCCl2 by using FT microwave spectroscopy. Because the molecular constants of a sufficient number of isotopomers were obtained, a complete structure determination was possible. They confirmed that the carbon was bonded to Cl:!. They also found that the bond distance is shortened by 0.02 A and the Cl-Cl distance is lengthened by 0.04 A in comparison to those distances in the monomers. Also, the disparity in the quadrupole coupling constants of the two chlorine atoms indicated that, upon complexation, the electron distribution of the inner C1 was much more strongly perturbed than that of the outer C1. Very few studies on complexes containing Br2 have been reported. To date, only the electronic spectra of HeBrZz2and NeBr223-24have been recorded by detecting the fluorescence associated with the B X transition of Br2. Both HeBr:! and NeBr:! are T shaped. The distances from the Br2 bond center to the rare gas atoms (Ro) are 3.8 and 3.67 A for He and Ne, respectively. Binding in the HeBr:! and NeBr:! (DO 73 cm-I) complexes is modest, and the complexes execute a large amplitude motion. Although OCC12 is linear, it is not to be expected that OCBr2 is necessarily linear. In the C 0 2 H X system, while C02HCl is linear, C02HBr25 is bent and the heavy atoms are essentially T shaped. The COX:! family could be similar to the COzHX family with a bent OCBr2 complex. Therefore, the objective of this study was to determine the geometry of the OCBrz complex. The natural abundances of Br79 and Br81 are 50.54% and 49.46%, respectively. Thus, four isotopic species should be observed with approximately equal intensities if the two bromine atoms are not equivalent. The other motivation for this study was to compare the bond strengths of OCBrz and 0CCl2. Even though infrared spectroscopy was among the first methods used in the study of weakly bound van der Waals molecules, no IR spectra of bromine complexes have been reported.
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0 1994 American Chemical Society
OCBr2 van der Waals Complex
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8311
As an extension of the OCCl2 work, we have obtained the IR spectra of all four naturally occurring isotopomers of OCBr2 in the 2143-cm-I C=O stretching region. Similar to 0CCl2,OCBrz is found to be linear. The bond strength in OCBrz is clearly stronger than that in OCClZ. The distances from the Brz bond center to the C O center of mass are approximately 4.884 and 4.893 A for the excited and ground states, respectively.
22
21
20
18
19
17
16
P
Experimental Section The detailed experimental arrangement has been described el~ewhere.2632~Only aspects relevant to this experiment are presented here. Experiments were carried out in a free jet expansion chamber. A mixture of 10% C O and 90% He/Ne (30:70) was used as a carrier. Bromine vapor was seeded into the carrier by passing the CO/He/Ne mixture over the surface of liquid bromine (Aldrich Chemical Co.) contained in a stainless steel vessel. To maintain the bromine vapor pressure at an acceptable level of 17 Torr, the vessel was kept at -23 OC in a CCld liquid slush bath. Prepurified C O and He/Ne were used without further purification. A free jet expansion was achieved by expanding the gas mixture through a pulsed slit nozzle located at the bottom of a vacuum chamber. The slit nozzle was 12.5 cm long and 0.25 pm wide and was pulsed by three solenoid valves (General Valve Series 9) which opened the slit for about 1 ms at a frequency of -3 Hz. The vacuum chamber was evacuated by a vapor booster pump backed by a blower and a mechanical pump. Background pressure in the chamber was about 10 mTorr during operation. Stagnation pressures behind the nozzle typically ranged from 1 to 2 atm. The jet expansion cooled the beam down to T,,, = 13 K. One mode from a tunable IR diode laser was selected by a 0.5-m monochromator. After exiting the monochromator, a small fraction of the laser radiation was sent through a NzO reference gas cell and a Fabry-Perot etalon to measure the absohte and relative frequencies. Most of the laser radiation entered the vacuum chamber and made eight passes through the molecular jet beam. I R absorption spectra were taken by rapidly scanning the laser frequency during a stable region of the gas pulse. To eliminate low-frequency noise, the signal was filtered through a band-pass filter with cutoff outside the frequency range of 10 to 100 kHz. Though peak intensities and linewidths were distorted, peak positions were retained. To eliminate thermal frequency drifts, the etalon and reference signals were taken simultaneously with the sample spectrum. Data were digitized by using three transient recorders and were stored in a 80386 PC. The spectra were then calibrated using the etalon marker and the N20 reference spectrum. Results The spectra were taken between 21 51.5 and 21 54.5 cm-I. As shown in Figure 1, new features were observed between the R( 1) and R(2) C O monomer lines. These new features were only seen in the expansion with Br2 and could be attributed to a COBr2 complex. The observed new features comprised consecutive clumps approximately equally spaced a t 0.044 cm-l intervals, consistent with the rotational constant of a linear OCBrz complex. The gap in the pattern, found at 2152.84 cm-I, was assigned as the band center, as indicated in Figure 2. The features on the red and blue sides of the band center were assigned to the P and R branches of OCBrz, respectively. No Q branch was observed. Therefore, we concluded that the OCBrz complex is linear. Another weak system with similar spacings (indicated as the U system in Figure 2) was observed in the expansion with high backing pressure. This system only occurred when Brz was included in the expansion mixture. Therefore, this band system must also belong to the same binary complex of C O and Brz, and it is probably a hot band. Other possible large complexes such as (Br2)zCO or (CO)*BrZ either would not be linear or would
I
219.9
2151.8
2152
I
I
2152.1
2151.2
Wavenumbers Figure 1. Portion of the P branch of the isotopomer-resolved spectrum of the OC(u”=O)-Br2 OC(d=l)-BrZ transition. The lengths of the lines at the top identify the different isotopomers.
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2152.5
218.6
2152.7
118.8
218.9
2153
2153 I
1153.2
Wavenumbers
Figure 2. Portion of the spectrum containing the band origin of the OC(v”=O)-Brz+OC(v’=l)-Br2 transition. TheU systemisunassigned.
have much smaller spacings in a linear configuration. Because this system is very weak and it severely overlapped the strong bands, it was not analyzed, and it will not be discussed further. In the P branch, the clumps with J > 8 were fully resolved into four peaks assignable to the 79-79, 79-81, 81-79, and 81-81 isotopomers where the first number refers to the bromine atom bound to the CO moiety. Two resolved peaks and one blended feature were observed from P(4) to P(8), and below J = 4, only broad unresolved features were seen. Judging from the proximity of the features in each clump, the origins for the bands assigned to each of the four isotopomers had to be very close to each other. Therefore, the relative positions for the different isotopomers in a clump might be determined by the rotational constant. A trial rotational constant calculation showed that the rotational constants should be ordered B(79-79) 5 B(81-79) > B(79-81) 5 B(81-81). Furthermore, substitution at the inner bromine, which is close to the center of mass, should not change the rotational constants by very much. Thus, the clumps should appear as two doublets a t high J. The spacings within the patterns should increase monotonically as J increases. Indeed, the clumps were gradually resolved as one moved away from the band center, and the P transitions at high J were resolved into two separate doublets. The shape and Jvariation of the isotopomer structure perfectly reflected the trial rotational constants. Therefore, the peaks were assigned accordingly as shown in Figure 1. For the R branch, two unresolved features were seen a t low J; two peaks were resolved from R(6) to R( 12), and four peaks were fully resolved for the clumps with J > 12. Similar to the P branch, the spacing between two doublets increased with J.
8312 The Journal of Physical Chemistry, Vol. 98, No. 34, 1994
Lin and Beaudet
TABLE 1: Experimental and Calculated Line Frequency for the Transition OC( v’=l)-Brz OC79Br79Bra transition
a
Vob
OC79Br81Br
OC*
2152.6655 2152.6219 2152.5783 2152.5344 2152.4905 2152.4466 2152.4027 2152.3587 21 52.3144 2152.2696 2152.2252 21 52.1806 2152.1362 2152.0908 21 52.0460 2152.001 1 2152.9558 2 151.9 104 21 51.8650 2151.8196 2151.7737 2151.7281 2151.6821 21 51.6362 2151.5898 2151.5436 2151.4975 2151.4509 21 51.4045
+1.5 +0.8 +0.4 -1.3 -1.1 -0.4 +0.9 +2.3 +3.0 -1.7 -0.1 +0.6 +3.8 -2.0 -0.5 +1.5 +0.4 -0.2 -0.2 +1.2 -1.7 +0.3 -1.2 +0.1 -2.9 -2.5 -0.3 -0.7 +1.0
2153.4296 2153.4709 21 53.5126 2153.5538 2153.5948 2153.6355 2 153.6763 2153.7170 2153.7577 21 53.7977 2153.8379 21 53.878 1 21 53.9186 2153.9581 21 53.9984 2154.0380 2154.0774 2154.1168 21 54.1 561 2154.1954 2154.2345 2154.2735 21 54.3 122
-1.1 -1.9 +2.1 +2.9 +2.4 +0.8 +0.8 +0.3 +1.8 -2.4 -3.4 -2.7 +1.2 -2.8 +1.6 +1.0 -0.5 -1.6 -1.6 +0.6 +1.0 +1.4 +0.9
Vob
OC( v“=O)-Brz
OC81Br79Br
oc
VOb
OC8IBr8lBr
oc
C
2152.4509 2152.4072 2152.3636 21 52.3197 2152.2755 21 52.23 18 2152.1875 2152.1429 2152.0989 2152.0542 21 52.0095 21 51.9646 21 51.9198 21 51.8746 2151.8294 21 5 1.7840 2151.7392 2151.6933 21 51.6477 21 5 1.6017 21 5 1S563 215 1.5103 21 51.4641 21 51.41 81 2153.1353 2153.1769 2153.2185 21 53.2602 2153.3015 2153.3430 2153.3845 2153.4253 21 53.4665 21 53.5074 2153.5482 2153.5888 2153.6294 21 53.6696 2153.7103 2153.7503 2153.7902 2153.8303 2153.8703 2153.9097 2153.9495 2153.9891 2154.0283 2154.0677 2154.1066 21 54.1456 21 54.1845 2154.2233 21 54.2619 21 54.3005
-2.6 -2.8 +2.1 -1.1 -2.8 +1.7 +1.3 -1.6 +2.5 +1.6 +1.3 +0.7 +1.3 -0.2 -1.5 -2.4 +2.9 -0.6 -1.1 -3.4 +o. 1 +0.5 -1.4 +1.4 +2.0 -0.1 -0.5 -0.2 -1.2 -0.4 +2.5 -0.6 +1.1 +0.5 +0.6 +0.3 +0.6 -1.8 +1.5
2152.4496 2152.4057 2152.3615 2152.3172 2152.2725 21 52.2283 21 52.1836 2152.1388 21 52.0938 2152.0490 2152.0042 215 1.9589 215 1.9135 215 1.8680 21 51.8224 21 51.7770 21 51.731 3 21 51.6852 21 51.6392 2151.5931 21 51.5472 215 1SO08 215 1.4544 2151.4078 21 53.1390 2153.1815 21 53.2236 2153.2656 21 53.3076 2153.3492 2153.3909 2153.4323 2153.4735 2153.5149 2153.5558 2153.5969 2153.6378 2153.6782 2153.7190 2153.7597 2153.7998 2 153.8404 2153.8804 21 53.9207 2 153.9602 21 53.9999 2154.0402 2154.0797 2154.1189 21 54.1582 2154.1975 2154.2366 2154.2755 21 54.3 145
+o.o
-2.7 -0.7 +1.0 -2.4 -0.2 +0.5 -0.8 +1.4 -0.9 -0.4 -0.0
+1.2 +1.3 +2.5
-0.6 +1.5 +1.9 +1.1 -2.2 +1.5 +o. 1 -0.2 -1.3 -0.2 +2.2 +0.9 -0.1 -0.4 -0.2 +0.3 +0.8 -1.4 -1.5 -1.8 +0.8
+o.o +o.o
-0.3 -4.0 -1.1 -0.8 +0.6 +1.8 +1.1 +2.1 +1.6 -0.0 +1.6 -0.0 +0.3 +0.7 -2.4 -1.2 +0.1 -2.6 +o. 1 -1.1 +1.6 -2.9 -3.5 +2.1 +1.5 -0.1 -0.8 +0.2 +1.6 +0.7 ,+3.2
Vob
2152.6708 2152.6278 2152.5844 2152.5412 21 52.4975 2152.4540 2152.4101 2152.3665 2152.3225 2152.2785 2152.2347 2152.1904 2152.1460 2152.1016 2152.0571 21 52.0124 2151.9677 2151.9227 2151.8777 21 5 1.8324 2151.7876 2151.7421 2151.6966 21 5 1.6509 21 51.6052 21 51.5595 2151.5139 2151.4676 21 5 1.4215
2153.4280 2153.4687 2153.5095 21 53.5504 2153.5910 2153.6315 21 53.6716 2 153.7120 2153.7524 2153.7924 2153.8324 2153.8723 21 53.9 120 2153.9514 21 53.9909 2154.0304 2154.0696 21 54.1086 21 54.1478 2154.1867 21 54.2254 21 54.2641 2154.3026
oc +1.9 +3.1 +1.3 +2.7
+o.o
+1.0 -1 .o +1.5 -0.2 -0.2 +2.8 +2.7 +1.5 +2.7 +2.9 +2.4 +2.8 +2.2 +1.9
+os +4.3 +3.1 +3.3 +0.3 +0.3 +0.9 +3.3 +0.6 +0.8
+5.4 +2.3 +l.S +3.2 +3.0 +2.6 -0.9 +0.7 +2.9 +1.4 +2.3 +2.6 +2.7
+os
+0.9 +2.5 +1.3 +0.2 +1.8 +1.6 +0.5 +0.6 -0.2
Error *0.0003 cm-l. X 0.0001 cm-I. Blended lines.
However, the spacing within the doublets decreased as Jincreased. Accordingly, the doublets away and near the band center could be assigned to the isotopomers with the outer bromine either Br79 and Bra1, respectively. However, the assignment within the doublets had to be reversed because the spacing decreased with J . Thus, the band origins for the different isotopomers had to be slightly displaced from each other. The frequencies of the resolved lines are listed in Table 1. Blended lines are not listed. The ground- and excited-state rotational constants (B”,B’) for these four isotopomers were first obtained independently by combination differences.28 Later. the molecular constants were
refined by least-squares fitting the data to a linear semirigid rotor model, u=
vo
+ B’J’(JY1) - D>(J’(J’+l))’B”J”(J”+l) + D’>(J”(J’’+1))2
(1)
where YO is the band origin and 0; and 0 ‘ ; are the centrifugal distortion constants of the excited and ground states, respectively. The molecular constants obtained from the fitting are listed in Table 2. The ground-state vibrational frequency for the van der Waals stretch of OCBr2was estimated by using o = (4B3/D)*/2.27
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8313
OCBr2 van der Waals Complex
TABLE 2
Molecular Constants and Bond Lengths of the OCBrz Complex OC79Br79Br OC79Br81Br
cm-I B”, cm-I B’, cm-l 0’5, cm-I D’I.cm-I w”, cm-1 YO.
_ I
w’,
cm-1
R’bw A R‘cM,A AR, A
R”o, 8, R‘o, A
2152.838 36(17) 0.021 5392(22) 0.021 4837(17) 0.58(17) X 10-8 0.70112) X 10-8 83 75 4.883 4.892 0 4.883 4.892 .
I
2152.8392 29(16) 0.021 3466(14) 0.021 2919(15) 0.40(10) X 10-8 0.53(10) X 10-8 98 85 4.898 4.907 -0.014 4.884 4.893 .
I
Assuming that the bond lengths of CO and Br2 remain unchanged upon complexation, the van der Waals bond distance can be calculated from the moments of inertia (I)by using the expression, Zoomplex = p R o 2 + I B , ~ ICO for the linear molecule. and ICO are the moments of inertia of the Br2 and CO monomers, respectively, and p is the reduced mass of OCBr2. The values of RCMfor the different isotopomers are listed in Table 2. Differences in RCMare due to the small shifts of the Br2 centers of mass upon isotopic substitution. The shift (AR) can be evaluated from
+
where ml and mz are the atomic masses of the two Br in the Br2 subunit. Correcting for AR,the distances from the Br2 bond center to the CO center of mass (Ro) become identical for all four isotopomers. Ro was found to be 4.8825(5) and 4.8925(5) A for the ground and excited states, respectively.
Discussion Spectra of the OCBr2 van der Waals complex are reported in this paper for the first time. The linear structure is consistent with a bonding scheme of electron donation from the HOMO of CO to the LUMO of Brz as was discussed for the case of OCC12.20 Although we do not have the necessary isotopic substitution on the CO required to determine the orientation of the CO in the complex, it is most likely that the orientation of OCBr2 is similar to that in OCC12, with the carbon pointing toward the halogen. Confirmation awaits a microwave study. The distances from the Br2 bond center to the CO center of mass are 4.884 and 4.893 A for ground and excited vibrational states, respectively. At first glance, the change in bond lengths suggests that the van der Waals bond strength is slightly weaker upon vibrational excitation of the CO subunit. However, the difference can be easily attributed to an apparent increase in the size of the CO subunit as the vibrational mode is excited. The expansion of the C O can be estimated from thevibrational wave functions. For a stretching frequency of 22 10 cm-I for CO, the maxima in the wave functions for the u = 0 and the u = 1 states occur at re and re+0.047 A, respectively. Also, the rms expectation values of r - re, (x2)1/2, are0.033 and 0.0585 A, respectively, indicating again an increase in the apparent size of the molecule by 0.025 A. Thus, the CO moiety should appear larger in the v = 1 state than in the v = 0 state by even more than the experimentally measured difference in RCM. Presuming the carbon is pointing directly toward Br2 in this complex, the bond distance of CBr would be 3.098 A. Because the bromine atomic radius is 0.15 A longer than the chlorine radius, one would expect the CBr bond distance to be longer also by 0.15 A. Surprisingly, the CBr bond distance is even slightly shorter than R c x l (3.12 A) in the OCCl2 complex. This shortening could have two possible explanations. Either the bromine complex is more strongly bonded than the corresponding chlorine complex, or, equally likely, a large amplitude bending
OCS1Br79Br
OCBIBrslBr
2152.841 17(15) 0.021 5299(13) 0.021 4738(13) 0.67(10) X 10-8 0.7319) . ,X 10-8 77 74 4.869 4.878 0.014 4.883 4.892
2152.842 09(13) 0.021 3353(13) 0.021 2793(15) 0.44(10) X 10-8 0.45(11) . , X 10-8 94 93 4.884 4.893 0 4.884 4.893
of the bromine moiety about the van der Waals bond draws the bromine center of mass closer to the CO. Since the bromine molecule is much heavier than the chlorine molecule, the frequency of this bending mode would be lower than and the amplitude larger than that in the corresponding chlorine complex. Since only parallel bands have been observed, only the coordinates along the symmetry axis have been determined and any molecular coordinates perpendicular to the axis cannot be obtained. The vibrational frequencies derived from the rotational and distortion constants are essentially the same for the different isotopomers with an average value of 88 cm-I. The variations are within the experimental error of the distortion constants, and any isotopic effect is hidden. The band origin of OCBr2 is found to be blue shifted approximately 9.57 cm-l relative to the uncomplexed monomer.29 This provides further insight into the relativevan der Waals bond strengths in the excited and ground vibrational states. The greater the band origin shift, the greater the effect of complexation on the CO moiety. A simple inspection of the energy level diagram along the van der Waals bond coordinate indicates that vcomp~ex - vmOnOmcr= DO“- DO’,where DO”and DO‘are the dissociation energies of the OCBr2 complex in the ground and excited uco vibrational states. The blue shift suggests that the bonding of the complex in the excited state is weaker than that of the corresponding ground state and that exciting the CO stretch weakens the van der Waals bond. Because the band origin shift is about 3.3 cm-’ greater than that of OCC123s(6.271 cm-I),* the bromine must interact with the CO more strongly than does C12. Thus, stronger bonding in OCBr2 is further substantiated. An alternative explanation can be based on a normal coordinate analysis. The blue shift in the band center means that the frequency of the C=O stretch has been increased and that the formation of a van der Waals complex strengthens or stiffens the C=O bond. In OCC12, Bunte et al. suggested that both the linear structure and the blue shift in the CEO stretching frequency can be explained by the Lewis acid-base nature of the carbonyl group.19320 In metal carbonyls, the carbon acts simultaneously as a base, donating a-electron density, and as a Lewis 7 acid, accepting ?r back-donation from the metal.30 Whether the CO frequency is lowered or raised depends on the relative contributions from the u bonding and the back-bonding. The HOMO in CO is the slightly antibonding 5 0 orbital that extends along the internuclear axis on the carbon end. In these halogen carbonyl complexes, this orbital donates electrons to the u*” LUMO which extends outward from the halogen molecule along its internuclear axis. Since bromine is known to have a higher electron affinity than chlorine,3’ it should attract more electron density from the carbonyl HOMO and it should 7 back-donate less to the CO. This explains the larger blue shift observed in the bromine than in the chlorine complex (9.57 vs 6.29 cm-1). Thus, a microwave study of the OCBr2 should determine that the C O bond length is shortened more than in OCCl2. Also, the bromine bond length in the complex should be longer than that in the isolated molecule because electron density is added to a halogen antibonding orbital.
8314 The Journal of Physical Chemistry, Vol. 98, No. 34, 1994
As seen in Table 2, the shifts in band origins for different isotopomers exhibit an interesting pattern with uo(81-81) > YO(81-79) > uo(79-81) > ~ ~ ( 7 9 - 7 9where ) the first number refers to the bromine closest to the CO group. These shifts originate from the relative offsets of the vibrational energy levels upon isotopic substitution. The band origin for different isotopomers will be the same if the vibrational energy levels in the ground and excited uco states are equally lowered. The energies of these states include the zero-point energies of the van der Waals and the Br-Br stretches. Since these shifts are small ( OCHCl > OCHBr > OCHI. In the OCX2 system, the relative strength of electron donation from the HOMO of the C O to the LUMO of the halogen is related more to the electron affinities of the halogen. Then we would expect the OCX2 bond strengths to follow the electronic affinity order Fz(3.08 eV) > Br2(2.55 eV) = 12(2.55 eV) > C11(2.38 eV).31
Lin and Beaudet This may explain why the binding strength and the band origin shift of OCBr2 are greater than those in OCC12. Another interaction that contributes markedly, when it is not overshadowed by other terms, is the polarizability. However, as in the C02HX and NzOHX series, the polarizability favors T-shaped complexes.
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