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Molecular structure of cadmium diiodide from combined electron diffraction and vibrational spectroscopic analysis
Volume199.number 5
CHEMICALPHYSICSLETTERS
13November I992
Molecular structure of cadmium diiodide from combined electron diffraction and vibrational spectroscopic analysis Natalja Vogt a,‘, Magdolna Hargittai b, Maria Kolonits b and Istvb Hargittai b*c * Sektionjir Spektren- und Strukturdokumentation, Universitiit Urn, Post&h 4066, W-7900 Urn, Germany b Structural Chemistry Research Group oftheHungarian Academy of Sciences, E&v& University, Postjiih I 17.1431 Budapest. Hungary c Institute for General and Analytical Chemistry, Budapest Technical University, 1521Budapest, Hungary Received 1 July 1992;in final form 24 August 1992
The molecular geometry and molecular dynamics of cdl2 was studied by joint electron diffraction/vibrational spectroscopic analysis.Thethermal average (rs) bond length is 2.582f 0.005A. The equilibrium configuration of tbe moleculeis linear and the equilibrium bond length is estimated to be 2.570f0.006 A.
1. Introduction
The investigation of metal dihalides has been a continuing project in the Budapest laboratory for a long time. The geometry of several first-row transition metal dihalides [ 1,2], alkaline earth dihalides [ 3,4] and group 12 dihalides [ 51 has been determined. Other group 12 dihalides were investigated by other laboratories,includingZnF, [ 6 1,CdCl, [ 71, CdBrz (81, HgCl, [9], and H& [lo]. In order to extend structural information on metal dihalides we have now carried out the investigation of cadmium diiodide. Determination of the geometry of these molecules is not alwayswithout controversy. High-temperature experimental conditions, coupled with large amplitude vibrations, make the determination of molecular shape by electron diffraction difficult. Additional information about the molecular shape from vibrational spectra and/or a joint electron diffraction/vibrational spectroscopic analysis greatly facilCorrespondence to: M. Hargittai, Structural Chemistry Research Group of the Hungarian Academy of Sciences, E6tvGs University, Postfach 117,1431 Budapest, Hungary. 1 Formerly Natalja Yu. Subbotina.
itates the reliable determination of the geometry of these molecules. 2. Experimental A commercial sample of 99.9% purity was used. The electron diffraction patterns were recorded in our modified EG- 1OOAapparatus [ 111 with a molybdenum nozzle [ 121. Recent mass spectrometric experiments showedno indication of dimeric species in the vapor of cadmium diiodide [ 131. Electron diffraction diagrams, 7 and 4 plates, taken at two camera ranges, 50 and 19 cm, respectively,have been analyzed. The data intervals were 2.000-14.000 A- ’ and 9.25-32.00 A-’ with 0.125 and 0.25 A-’ data steps, for the 50 and 19 camera ranges, respectively. The nozzle temperature was 678 K. Other experimental conditions and data processingwere the same as in our other studies. Listingsof total experimental electron diffraction intensities are availablefrom the authors upon request. The electron scattering factors were taken from the usual sources [ 14,151.The molecular intensities and radial distributions are shown in figs. 1 and 2.
0009-2614/92/S 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
441
CHEMICALPHYSICSLETTERS
Volume 199,number 5
13 November 1992
Table 1 Thermal averagestructural parameters of CdI, a) sM(s
Cd-I rr (A) I (A) L (A) b, K (A’) 4 (A) Gdk (A) Ye(cm-‘) Cl a (A-‘) d, I
0
5
10
15
20
25
30
s,i-’
Fig. 1. Experimental ( * * I ) and theoretical (-) intensity curves for Cd12for two camera ranges.
molecular
CdIz
I
0
, 1
2
3
4
5 6 r,A
7
Fig. 2. Experimental (- - -) and theoretical (-) radial distributions for CdI2.The first peak corresponds to the Cd-I bond distance and the second to the I...1non-bonded distance.
3. Analysisand results Conventional electron difraction analysis. The electron diffraction analysis, in agreement with a previous separate mass spectrometricstudy [ 131, did not show any indication of dimetic speciesin the vapor. This is also in agreement with our earlier experience with group 12dihalides [ 5 1, The two peaks of the radial distribution curve can be assignedto the Cd-I bond distance and to the I...1non-bonded distance. The final structural parameters from a leastsquares refinement, based on the molecular intensities, are listed in table 1.
442
2.582+0.005 0.075&0.002 0.0075 1.84x 1O-5+4.8x lO-‘j
I...1 5.108+0.011 0.118~0.004 0.102 0.056f 0.004 0.061
53*4 1.2iO.4
*) Indicated are total errors, calculated according to the expression: u,= (2& toLc) ‘12,where om is the least-squaresstandard deviation and the scale errors were taken to be 0.2%,2%, 4%, and 20% for distances, amplitudes, force constants, and anhannonicity parameters, respectively. b, The “calculated” values correspond to the experimental frequencies, 155.1 [16], 51.0 [17] and261.3 (171 cm-‘forv,, v2,and v,, respectively. ‘) Calculatedfrom the experimental shrinkage,S, ‘) Morse constant, calculatedfrom K, see, e.g, ref. [ 11.
Cadmium diiodide was investigated by visual electron diffraction several decades ago [ 18-201 and the followingbond lengthswere reported: 2.60+ 0.02 A [18],2.56+0.03A [19] and2.55+0.02A [20]. The new bond length (r,=2.582+0.005 8,) is in agreement with the oldest two data consideringtheir large uncertainties. Sinceall three vibrational frequenciesof CdI, have been determined experimentally [ 16,17,21,22] a normal-coordinate analysiswas also carried out. The calculated vibrational amplitudes are also listed in table 1. As observed for other similar linear systems with considerable nonrigidity and/& high temperature experiments [ 5,231,the calculatedI( [...I) value is smaller than the experimental one indicating that the harmonic approximation, used in the traditional normal-coordinate analysis, does not describe well the stretching vibrations of these molecules. The electron diffraction geometry is a set of average internuclear distances with averaging over all molecular vibrations at the experimental temperature [ 241. Due to the so-calledshrinkageeffect symmetrical triatomic molecules will appear to be bent even if their equilibrium configuration is linear. This is the reason why the equilibrium configuration of these molecules cannot be determined from electron diffraction alone. Conversely having information
Volume 199,number 5
13November 1992
CHEMICALPHYSICSLETTERS
Table 2 Vibrational frequencies from the joint electron diffraction/vibrational spectroscopic analyses Frequency (cm-‘)
Initial valuesa)
Origin of initial values
Anharmonic approximationb,
Semirigid model ‘)
Vl
155.1(5.0) 51.0( 1.0) 261.3( 1.0)
Rr@, [ 161 WY 1171 gas, 1171
148+5 52*2
154+6
262?10
261+11
v2
V3
‘) Valuesin parentheses are estimated uncertainties. For u, the uncertainty is estimated on the basis of the difference between the available gas phase and matrix values. The uncertainty for v2and v1is estimated as twice the resolution of the gas-phaseexperiment. b, The estimated total errors of the frequencies were calculated from the total errors of the force constants (and of the bond distance in case of u2) using error propagation. ‘) Kr matrix, Raman spectroscopy. Table 3 Results of the joint electron diffraction/vibrational spectroscopic analyses Parameter
Anharmonic approximation a)
r: (A) r,” (A) r:” (A) J (m&n/A)
2.569+ 0.005
1.6OiO.08 0.03i 0.04 -2.96k0.86 0.03OkO.002
f, (mdyn/AI f, @@a/A) f, (mdyn/A) k2 (mdyn/A) k4 (mdyn/A)
Semirigid model a’ 2.572f0.006b’ 2.583+ 0.005 1.67kO.08 0.11~0.04
0.22* 0.11 0.00~0.17
‘) For error estimation see footnote a) of table 1. b, Calculated from r$ using the expression rg-rEh= [ 2fa/ cf,t f,) 16and applyingMorse-typeanharmoniccorrections [ 4,261. This estimated valuecoincides,within experimentalerror, with r: estimated from r,,which is 2.570+0.006 A.
about the molecular symmetry from other sources, the observation of shrinkage can be utilized to estimate the bending vibrational frequency. Our estimate for Cd12,53 + 4 cm-’ agreeswellwith the spectroscopic value [ 171 within experimental error. As indicated in our previous paper [ 11,the Morse constant, a, can be estimated from the electron diffraction anharmonicity parameter, K,especiallyif the electron diffraction intensitiesare availablefor a long s range. Our estimate for this parameter is a=1.2&0.4
A-‘.
Prior experience with non-rigid molecules shows that the thermal average bond length, r*, and the equilibriumbond length, r,, differ severaltimes more than the uncertainty of their determination [ 4,251. Therefore it is useful to estimate the equilibrium
bond length of these types of molecules if possible. It was shown recently [ 1,4] that the equilibrium bond length of these simple linear triatomic molecules can be estimated well using Morse-type vibrational corrections [ 261. The value of this r? parameter is 2.570f 0.006 A. In calculatingthe uncertainty of this parameter, the errors of rg and of the three types of anharmonicity correction terms were taken into account [ 4 1. Joint electron dijj5ractionlvibrational spectroscopic analysis. The application of this method may greatly
enhance the reliability of the determination of the moleculargeometryand moleculardynamicsof small non-rigid molecules.We have shown [ 1] that in this way the molecular geometry can be determined unambiguously even in the absence of complete independent spectroscopicinformation. The case of Cd& is more advantageous, however, since all its frequencies are available from spectroscopy (see table 2). Even here the method is still useful for the determination of the equilibrium bond length. According to our earlier experience [ 1 ] a model potential in the anharmonic approximation can be expected to perform best for such purpose. The results of this analysis are given in table 3. The r: value determined this way is the same as the one calculated by simple Morse-type vibrational corrections from the electron diffraction rgvalue. The Morse constant, estimated from the cubic force constant to be a= 1.4kO.5 A-‘, coincides, within experimental error, with the value determined from the anharmonicity constant (IE) of the molecular intensity expression. The so-called “semirigid model”, introduced by Gershikov and Spiridonov [27] and applied suc443
Volume 199,number 5
CHEMICALPHYSICSLETTERS
cessfully to the first-row transition metal dihalides [ 1,231, can be used to determine the molecularshape of these molecules.As expected, Cd12appears to be linear; all attempts to produce a bent model approximating well the experimental data failed. The parameters of the force field given by this model are given in table 3. As already indicated before [ 251, the bond length correspondingto this model, usually calledan r$ type distance,does,not approximatewell the equilibrium bond distance; therefore it should not be used as its substitute. The rr parameter can be estimated from this distance type as well introducing anharmonicity corrections. It is the same, within experimental error, as the r,” parameter estimated from the rgdistance (2.572 f 0.006 A versus 2.570+0.006 A from rzh and r,, respectively). The r: and r: type bond lengths also coincide and are believed to be a good representation of the true equilibrium bond distance of cadmium diiodide.
4. Discussion The variation of gas-phasebond lengths of group 12 diiodides follows the trend observed earlier for MX2 derivatives of group 12 metals by Haaland et al. [ 7 1. While there is a substantial increase in bond lengths when going from the zinc dihalides to the corresponding cadmium dihalides, the bond lengths of mercury dihalides appear to be somewhat shorter than those of cadmium dihalides. It has been suggested [ 7 ] that the origin of this anomaly is a relativistic contraction of the Hg s orbitals.
Acknowledgement One of us (NV) gratefully acknowledgesfinancial support by Dr. Barbara Mez-Starck,former head of the Documentation Center in Ulm and the kind assistance of Dr. Jllrgen Vogt.
444
13November 1992
References [ 1] M. Hargittai,N.Yu.Subbotina,M. Kolonits and I. Hargittai, J. Chem. Phys. 94 (1991) 7278. 121M. Hargittai, Coord. Chem. Rev. 91 (1988) 35. [3] E. Vajda, M. Hargittai, I. Hargittai, J. Tremmel and J. Brunvoll,Inorg. Chem. 26 (1987) 1171. [4] M. Hargittai, M. Kolonits, D. Knausz and I. Hargittai, J. Chem. Phys. 96 (1992) 8980. [5] M. Hargittai, J. Tremmel and I. Hargittai, Inorg. Chem. 25 (1986) 3163. [6] G.V. Girichev, A.G. Gershikov and N.Yu. Subbotina, Zh. Sttukt. Khim. 29No. 6 (1988) 139. [7] A. Haaland, K.-G. Martinsen and J. Tremmel, Acta Chem. Stand. 46 (1992), in press. [S] V.M. Petrov, A.N. Utkin, G.V. Gitichev and A.A. Ivanov Zh. Strukt. Khim. 26 No. 2 (1985) 52. [9] K. Kashiwabara, S. Konaka and M. Kimura, Bull. Chem. &.Japan46(1973)410. [lo] V.P. Spiridonov,A.G. Gershikov and B.S.Butayev,J. Mol. Struct. 52 (1979) 53. [ 111I. Hargittai, J. Tremmel and M. Kolonits, HSI Hung. Sci. Instr. 50 (1980) 31. [ 121J. Tremmeland I. Hargittai, J. Phys. E 18 (1985) 148. [ 131K. SkudIarsti,J. Dudekand J. Kapala,J. Chem.Thennodyn. 19 (1987) 857. [ 141R.A. Bonham and L. Schiifer,International tables for X-ray crystallography,Vol.4 (Kynoch,Birmingham,1974)p. 176. [ 151C. Tavard,D. Nicolasand M. Rouault,J. Chim. Phys. PhysChim. Biol. 64 ( 1967) 540. [ 161A. Strull, A. Givan and A. Loewenschuss,J. Mol. Spectry. 62 (1976) 283. [17]R.J.M. Konings, AS. Booij and E.H.P. Cordfunke, Vib. Sp-ectry.2 (1992) 251. [ 1810. Hasseland L.C.Stmmme, 2. Physik. Chem.B 38 (1938) 466. [ 191M.W.Listerand L.E.Sutton,Trans. FaradaySot. 37 (1941) 406. [2O]P.A. Akishin, VP. Spiridonov, V.A. Naumov and N.G. Rambidi, Zh. Fiz. Khim. 30 (1956) 155. 1211A. Givan and A. Loewenschuss, Spectmchim. Acta A 34 (1978) 765. [22] G.K. Selivanov,NJ. Zavalishin and A.A.Mal’tsev,Vestnik Mosk.Univ.Ser.Khim.28 (1972) 712. [23] A.G. Gershikov,N.Yu. Subbotina and G.V. Girichev, Zh. Strukt. Khim. 27 No. 5 (1986) 36. [24]K. Kuchitsu, in: Accurate molecular structures, eds. A. Domenicano and I. Hargittai (Oxford Univ. Press, Oxford, 1992). [25] M. Hargittai and I. Hargittai, Int. J. Quantum Chem., in press. [26] L.S. Bat-tell,J. Chem. Phys. 70 (1979) 4581. [27] A.G. Gershiiov and V.P. Spiridonov,Zlt. Strukt. Kbim. 27 No. 5 (1986) 30.
CHEMICALPHYSICSLETTERS
13November I992
Molecular structure of cadmium diiodide from combined electron diffraction and vibrational spectroscopic analysis Natalja Vogt a,‘, Magdolna Hargittai b, Maria Kolonits b and Istvb Hargittai b*c * Sektionjir Spektren- und Strukturdokumentation, Universitiit Urn, Post&h 4066, W-7900 Urn, Germany b Structural Chemistry Research Group oftheHungarian Academy of Sciences, E&v& University, Postjiih I 17.1431 Budapest. Hungary c Institute for General and Analytical Chemistry, Budapest Technical University, 1521Budapest, Hungary Received 1 July 1992;in final form 24 August 1992
The molecular geometry and molecular dynamics of cdl2 was studied by joint electron diffraction/vibrational spectroscopic analysis.Thethermal average (rs) bond length is 2.582f 0.005A. The equilibrium configuration of tbe moleculeis linear and the equilibrium bond length is estimated to be 2.570f0.006 A.
1. Introduction
The investigation of metal dihalides has been a continuing project in the Budapest laboratory for a long time. The geometry of several first-row transition metal dihalides [ 1,2], alkaline earth dihalides [ 3,4] and group 12 dihalides [ 51 has been determined. Other group 12 dihalides were investigated by other laboratories,includingZnF, [ 6 1,CdCl, [ 71, CdBrz (81, HgCl, [9], and H& [lo]. In order to extend structural information on metal dihalides we have now carried out the investigation of cadmium diiodide. Determination of the geometry of these molecules is not alwayswithout controversy. High-temperature experimental conditions, coupled with large amplitude vibrations, make the determination of molecular shape by electron diffraction difficult. Additional information about the molecular shape from vibrational spectra and/or a joint electron diffraction/vibrational spectroscopic analysis greatly facilCorrespondence to: M. Hargittai, Structural Chemistry Research Group of the Hungarian Academy of Sciences, E6tvGs University, Postfach 117,1431 Budapest, Hungary. 1 Formerly Natalja Yu. Subbotina.
itates the reliable determination of the geometry of these molecules. 2. Experimental A commercial sample of 99.9% purity was used. The electron diffraction patterns were recorded in our modified EG- 1OOAapparatus [ 111 with a molybdenum nozzle [ 121. Recent mass spectrometric experiments showedno indication of dimeric species in the vapor of cadmium diiodide [ 131. Electron diffraction diagrams, 7 and 4 plates, taken at two camera ranges, 50 and 19 cm, respectively,have been analyzed. The data intervals were 2.000-14.000 A- ’ and 9.25-32.00 A-’ with 0.125 and 0.25 A-’ data steps, for the 50 and 19 camera ranges, respectively. The nozzle temperature was 678 K. Other experimental conditions and data processingwere the same as in our other studies. Listingsof total experimental electron diffraction intensities are availablefrom the authors upon request. The electron scattering factors were taken from the usual sources [ 14,151.The molecular intensities and radial distributions are shown in figs. 1 and 2.
0009-2614/92/S 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
441
CHEMICALPHYSICSLETTERS
Volume 199,number 5
13 November 1992
Table 1 Thermal averagestructural parameters of CdI, a) sM(s
Cd-I rr (A) I (A) L (A) b, K (A’) 4 (A) Gdk (A) Ye(cm-‘) Cl a (A-‘) d, I
0
5
10
15
20
25
30
s,i-’
Fig. 1. Experimental ( * * I ) and theoretical (-) intensity curves for Cd12for two camera ranges.
molecular
CdIz
I
0
, 1
2
3
4
5 6 r,A
7
Fig. 2. Experimental (- - -) and theoretical (-) radial distributions for CdI2.The first peak corresponds to the Cd-I bond distance and the second to the I...1non-bonded distance.
3. Analysisand results Conventional electron difraction analysis. The electron diffraction analysis, in agreement with a previous separate mass spectrometricstudy [ 131, did not show any indication of dimetic speciesin the vapor. This is also in agreement with our earlier experience with group 12dihalides [ 5 1, The two peaks of the radial distribution curve can be assignedto the Cd-I bond distance and to the I...1non-bonded distance. The final structural parameters from a leastsquares refinement, based on the molecular intensities, are listed in table 1.
442
2.582+0.005 0.075&0.002 0.0075 1.84x 1O-5+4.8x lO-‘j
I...1 5.108+0.011 0.118~0.004 0.102 0.056f 0.004 0.061
53*4 1.2iO.4
*) Indicated are total errors, calculated according to the expression: u,= (2& toLc) ‘12,where om is the least-squaresstandard deviation and the scale errors were taken to be 0.2%,2%, 4%, and 20% for distances, amplitudes, force constants, and anhannonicity parameters, respectively. b, The “calculated” values correspond to the experimental frequencies, 155.1 [16], 51.0 [17] and261.3 (171 cm-‘forv,, v2,and v,, respectively. ‘) Calculatedfrom the experimental shrinkage,S, ‘) Morse constant, calculatedfrom K, see, e.g, ref. [ 11.
Cadmium diiodide was investigated by visual electron diffraction several decades ago [ 18-201 and the followingbond lengthswere reported: 2.60+ 0.02 A [18],2.56+0.03A [19] and2.55+0.02A [20]. The new bond length (r,=2.582+0.005 8,) is in agreement with the oldest two data consideringtheir large uncertainties. Sinceall three vibrational frequenciesof CdI, have been determined experimentally [ 16,17,21,22] a normal-coordinate analysiswas also carried out. The calculated vibrational amplitudes are also listed in table 1. As observed for other similar linear systems with considerable nonrigidity and/& high temperature experiments [ 5,231,the calculatedI( [...I) value is smaller than the experimental one indicating that the harmonic approximation, used in the traditional normal-coordinate analysis, does not describe well the stretching vibrations of these molecules. The electron diffraction geometry is a set of average internuclear distances with averaging over all molecular vibrations at the experimental temperature [ 241. Due to the so-calledshrinkageeffect symmetrical triatomic molecules will appear to be bent even if their equilibrium configuration is linear. This is the reason why the equilibrium configuration of these molecules cannot be determined from electron diffraction alone. Conversely having information
Volume 199,number 5
13November 1992
CHEMICALPHYSICSLETTERS
Table 2 Vibrational frequencies from the joint electron diffraction/vibrational spectroscopic analyses Frequency (cm-‘)
Initial valuesa)
Origin of initial values
Anharmonic approximationb,
Semirigid model ‘)
Vl
155.1(5.0) 51.0( 1.0) 261.3( 1.0)
Rr@, [ 161 WY 1171 gas, 1171
148+5 52*2
154+6
262?10
261+11
v2
V3
‘) Valuesin parentheses are estimated uncertainties. For u, the uncertainty is estimated on the basis of the difference between the available gas phase and matrix values. The uncertainty for v2and v1is estimated as twice the resolution of the gas-phaseexperiment. b, The estimated total errors of the frequencies were calculated from the total errors of the force constants (and of the bond distance in case of u2) using error propagation. ‘) Kr matrix, Raman spectroscopy. Table 3 Results of the joint electron diffraction/vibrational spectroscopic analyses Parameter
Anharmonic approximation a)
r: (A) r,” (A) r:” (A) J (m&n/A)
2.569+ 0.005
1.6OiO.08 0.03i 0.04 -2.96k0.86 0.03OkO.002
f, (mdyn/AI f, @@a/A) f, (mdyn/A) k2 (mdyn/A) k4 (mdyn/A)
Semirigid model a’ 2.572f0.006b’ 2.583+ 0.005 1.67kO.08 0.11~0.04
0.22* 0.11 0.00~0.17
‘) For error estimation see footnote a) of table 1. b, Calculated from r$ using the expression rg-rEh= [ 2fa/ cf,t f,) 16and applyingMorse-typeanharmoniccorrections [ 4,261. This estimated valuecoincides,within experimentalerror, with r: estimated from r,,which is 2.570+0.006 A.
about the molecular symmetry from other sources, the observation of shrinkage can be utilized to estimate the bending vibrational frequency. Our estimate for Cd12,53 + 4 cm-’ agreeswellwith the spectroscopic value [ 171 within experimental error. As indicated in our previous paper [ 11,the Morse constant, a, can be estimated from the electron diffraction anharmonicity parameter, K,especiallyif the electron diffraction intensitiesare availablefor a long s range. Our estimate for this parameter is a=1.2&0.4
A-‘.
Prior experience with non-rigid molecules shows that the thermal average bond length, r*, and the equilibriumbond length, r,, differ severaltimes more than the uncertainty of their determination [ 4,251. Therefore it is useful to estimate the equilibrium
bond length of these types of molecules if possible. It was shown recently [ 1,4] that the equilibrium bond length of these simple linear triatomic molecules can be estimated well using Morse-type vibrational corrections [ 261. The value of this r? parameter is 2.570f 0.006 A. In calculatingthe uncertainty of this parameter, the errors of rg and of the three types of anharmonicity correction terms were taken into account [ 4 1. Joint electron dijj5ractionlvibrational spectroscopic analysis. The application of this method may greatly
enhance the reliability of the determination of the moleculargeometryand moleculardynamicsof small non-rigid molecules.We have shown [ 1] that in this way the molecular geometry can be determined unambiguously even in the absence of complete independent spectroscopicinformation. The case of Cd& is more advantageous, however, since all its frequencies are available from spectroscopy (see table 2). Even here the method is still useful for the determination of the equilibrium bond length. According to our earlier experience [ 1 ] a model potential in the anharmonic approximation can be expected to perform best for such purpose. The results of this analysis are given in table 3. The r: value determined this way is the same as the one calculated by simple Morse-type vibrational corrections from the electron diffraction rgvalue. The Morse constant, estimated from the cubic force constant to be a= 1.4kO.5 A-‘, coincides, within experimental error, with the value determined from the anharmonicity constant (IE) of the molecular intensity expression. The so-called “semirigid model”, introduced by Gershikov and Spiridonov [27] and applied suc443
Volume 199,number 5
CHEMICALPHYSICSLETTERS
cessfully to the first-row transition metal dihalides [ 1,231, can be used to determine the molecularshape of these molecules.As expected, Cd12appears to be linear; all attempts to produce a bent model approximating well the experimental data failed. The parameters of the force field given by this model are given in table 3. As already indicated before [ 251, the bond length correspondingto this model, usually calledan r$ type distance,does,not approximatewell the equilibrium bond distance; therefore it should not be used as its substitute. The rr parameter can be estimated from this distance type as well introducing anharmonicity corrections. It is the same, within experimental error, as the r,” parameter estimated from the rgdistance (2.572 f 0.006 A versus 2.570+0.006 A from rzh and r,, respectively). The r: and r: type bond lengths also coincide and are believed to be a good representation of the true equilibrium bond distance of cadmium diiodide.
4. Discussion The variation of gas-phasebond lengths of group 12 diiodides follows the trend observed earlier for MX2 derivatives of group 12 metals by Haaland et al. [ 7 1. While there is a substantial increase in bond lengths when going from the zinc dihalides to the corresponding cadmium dihalides, the bond lengths of mercury dihalides appear to be somewhat shorter than those of cadmium dihalides. It has been suggested [ 7 ] that the origin of this anomaly is a relativistic contraction of the Hg s orbitals.
Acknowledgement One of us (NV) gratefully acknowledgesfinancial support by Dr. Barbara Mez-Starck,former head of the Documentation Center in Ulm and the kind assistance of Dr. Jllrgen Vogt.
444
13November 1992
References [ 1] M. Hargittai,N.Yu.Subbotina,M. Kolonits and I. Hargittai, J. Chem. Phys. 94 (1991) 7278. 121M. Hargittai, Coord. Chem. Rev. 91 (1988) 35. [3] E. Vajda, M. Hargittai, I. Hargittai, J. Tremmel and J. Brunvoll,Inorg. Chem. 26 (1987) 1171. [4] M. Hargittai, M. Kolonits, D. Knausz and I. Hargittai, J. Chem. Phys. 96 (1992) 8980. [5] M. Hargittai, J. Tremmel and I. Hargittai, Inorg. Chem. 25 (1986) 3163. [6] G.V. Girichev, A.G. Gershikov and N.Yu. Subbotina, Zh. Sttukt. Khim. 29No. 6 (1988) 139. [7] A. Haaland, K.-G. Martinsen and J. Tremmel, Acta Chem. Stand. 46 (1992), in press. [S] V.M. Petrov, A.N. Utkin, G.V. Gitichev and A.A. Ivanov Zh. Strukt. Khim. 26 No. 2 (1985) 52. [9] K. Kashiwabara, S. Konaka and M. Kimura, Bull. Chem. &.Japan46(1973)410. [lo] V.P. Spiridonov,A.G. Gershikov and B.S.Butayev,J. Mol. Struct. 52 (1979) 53. [ 111I. Hargittai, J. Tremmel and M. Kolonits, HSI Hung. Sci. Instr. 50 (1980) 31. [ 121J. Tremmeland I. Hargittai, J. Phys. E 18 (1985) 148. [ 131K. SkudIarsti,J. Dudekand J. Kapala,J. Chem.Thennodyn. 19 (1987) 857. [ 141R.A. Bonham and L. Schiifer,International tables for X-ray crystallography,Vol.4 (Kynoch,Birmingham,1974)p. 176. [ 151C. Tavard,D. Nicolasand M. Rouault,J. Chim. Phys. PhysChim. Biol. 64 ( 1967) 540. [ 161A. Strull, A. Givan and A. Loewenschuss,J. Mol. Spectry. 62 (1976) 283. [17]R.J.M. Konings, AS. Booij and E.H.P. Cordfunke, Vib. Sp-ectry.2 (1992) 251. [ 1810. Hasseland L.C.Stmmme, 2. Physik. Chem.B 38 (1938) 466. [ 191M.W.Listerand L.E.Sutton,Trans. FaradaySot. 37 (1941) 406. [2O]P.A. Akishin, VP. Spiridonov, V.A. Naumov and N.G. Rambidi, Zh. Fiz. Khim. 30 (1956) 155. 1211A. Givan and A. Loewenschuss, Spectmchim. Acta A 34 (1978) 765. [22] G.K. Selivanov,NJ. Zavalishin and A.A.Mal’tsev,Vestnik Mosk.Univ.Ser.Khim.28 (1972) 712. [23] A.G. Gershikov,N.Yu. Subbotina and G.V. Girichev, Zh. Strukt. Khim. 27 No. 5 (1986) 36. [24]K. Kuchitsu, in: Accurate molecular structures, eds. A. Domenicano and I. Hargittai (Oxford Univ. Press, Oxford, 1992). [25] M. Hargittai and I. Hargittai, Int. J. Quantum Chem., in press. [26] L.S. Bat-tell,J. Chem. Phys. 70 (1979) 4581. [27] A.G. Gershiiov and V.P. Spiridonov,Zlt. Strukt. Kbim. 27 No. 5 (1986) 30.