10095
J. Phys. Chem. 1994, 98, 10095-10097
Iodine Azide. Molecular Structure from Gas-Phase Electron Diffraction Magdolna Hargittai,.'la Judit Molnhr,la Thomas M. Klap8tke,lb Inis C. Tornieporth-Oetting,lb Mhria Kolonits,'B and Istvhn HargittailafC Structural Chemistry Research Group of the Hungarian Academy of Sciences, Eotvos University, P$ I 17, H-1431 Budapest, Hungary, Institut f i r Anorganische und Analitische Chemie der Technischen Universitat, Strasse des 17. Juni 135, 0-10623 Berlin, Germany, and Institute of General and Analytical Chemistry, Budapest Technical University, H-1521 Budapest, Hungary Received: May 17, 1994; In Final Form: July 21, 1994@
The structure of the free iodine azide molecule was determined by gas-phase electron diffraction. The trans bent geometry determined from the experiment is in agreement with the prediction of MO calculations and with the structure of the rest of the halogen azide series. The following parameters (rg bond lengths, A; r, angles, degrees) characterize the geometry of the iodine azide molecule: (N-N)" 1.204 f 0.004, A(") 0.113 f 0.022, I-N 2.120 f 0.010, N-N-N 169.6 f 3.0, and I-N-N 106.6 f 1.1.
Introduction
This work is a continuation of our previous studies of the iodine azide structure2 and of our electron diffraction structure analysis of bromine azide.3 Iodine azide was the last among the halogen azides whose gas-phase molecular geometry has not been determined by experiment. Ab initio MO calculat i o n ~employing ~,~ various basis sets have consistently predicted a trans bent geometry and its parameters. Experimental Section It was a special problem to produce the iodine azide vapor in the electron diffraction experiment safely and direct it across the electron beam. IN3 was synthesized according to eq a: AgN,(solid)
+ I,(solution)
CFCl,, 0 "C,1 h IN,(solution)
+ AgI(so1id) (a)
CAUTION: IN3 is explosive! The explosive nature increases with greater purity. Since I N 3 is highly explosive and therefore the handling is difficult, several precautions were taken to make the experiment as safe and successful as possible. In particular, (i) the scale of each preparation was reduced to an amount that was sufficient for the exposure of only a few electron diffraction plates and (ii) the preparation of AgN3 and I N 3 and the handling of IN3 including the connection of the sample container to the electron diffraction device were carried out by one chemist from the Berlin group, who is used to handling shock-sensitive and highly explosive materials. I N 3 was prepared by the reaction of an excess of dry, freshly prepared silver azide, AgN3 (from AgN03 and NaN3, both Aldrich), and iodine (Aldrich) in CFCl3 (Merck) solution. At 0 "C 0.12 g (0.80 m o l ) of AgN3 was suspended (magnetic bar) in a PTFE (poly(tetrafluoroethy1ene))beaker in 10 mL of CFC13, and then 0.05 g (0.20 m o l ) of IZwas added to the stirred suspension. The reaction mixture was stirred at 0 "C and allowed to react for 1 h. Since both AgN3 (excess) and AgI are insoluble in CFC13, the clear yellow IN&FC13 solution was poured into the PTFE sample container (which can be
* Abstract published in Advance ACS Abstracts, September 1, 1994. 0022-3654/94/2098-10095$04.50/0
b
4
8
12
16
20
24
s,A-'
Figure 1. Electron diffraction molecular intensities from the molecular beam of the gaseous product from the reaction of iodine and silver azide. Conditions: 60 kV electrons, camera distances 50 and 19 cm, sample container and nozzle at room temperature. M(s) = molecular intensity, s = (42731)sin(W2) in A-l, E = experimental,T = theoretical, and A = difference curves. Due to the large noise of the final region of the 50 cm camera-range data, the intensity values over s = 9.25 A-' were given a 0.25 weight in the refinement. The corresponding part of the difference curve is indicated by broken lines.
connected to the electron diffraction inlet system). The sample container was placed into a precooled polyethylene vacuum desiccator, and the solvent (CFC13) was pumped off at 0 "C and 30 Torr (Brandt membrane vacuum pump). In the next step the sample container was cooled to -78 "C (dry ice/ acetone), and all material volatile at this temperature was pumped off in a dynamic vacuum. For the electron diffraction experiment the sample container was allowed to slowly warm up to room temperature and then the electron diffraction patterns were recorded. The other experimental conditions and the data reduction were similar to those of the bromine azide study.3 Analysis The electron diffraction molecular intensities from experiments at two camera ranges are shown in Figure 1. The radial distribution obtained from the combined intensities is shown in Figure 2. There is a relatively large amount of noise on the intensity distributions and considerable difference curve on the 0 1994 American Chemical Society
10096 J. Phys. Chem., Vol. 98, No. 40, 1994
Hargittai et al. TABLE 1: Bond Lengths, r (A), and Bond Angles, ra (deg), of IN3 Determined by Electron Diffraction (ED)' and Corresponding re from ab Initio calculation^^^^ for Comparison
f(r)
n 1
2
3
,
4
5
r,A Figure 2. Radial distribution,flr),obtained from the electron diffraction
intensities combined from the two camera ranges (cf. Figure 1). E = experimental,T = theoretical, and A = difference curves. The heights of vertical bars are roughly proportional to the relative weights of contributions of internuclear distances r to the electron scattering. radial distributions. The presence of iodine, a heavy element, in iodine azide causes a strong atomic scattering. This is compounded with the possibility of the presence of various contaminants, such as unreacted 12, the solvent CFCl3, and possible side products, HI and HOI. Their presence has been tested in the refinements, and up to 4% of HI and CFCl3 and up to 1% of HOI and 1 2 could not be excluded. This possible presence of contaminant species has been taken into account in the error estimation of the parameters. The mean amplitudes of vibration ( I ) were calculated by a normal coordinate analysis5 based on the measured frequencies2 except for the G(I") bending mode, for which a calculated value4 was used. For room temperature the following 1 values (angstroms) were obtained and then used as assumed parameters in the least-squares refinements: I-N 0.0648, Nl-N2 0.0437, N2-N3 0.0346, I..-N2 0.0790, I.**N30.1069, Nl-**N30.0466. All of the vibrational amplitudes, except the two I(N-N) amplitudes, could be refined without causing appreciable change in the other structural parameters, but due to the relatively large noise of the experimental data, the use of calculated amplitudes was preferred. Since the two different N-N bond distances could not be refined simultaneously, their mean value and difference were refined. The relatively large error of the A(") parameter makes the determination of the individual nitrogen-nitrogen bond distances rather uncertain. However, even if we take the average of the two c a l c ~ l a t e dvalues ~ ~ ~ for A("), 0.091 A, and use it as a constraint in the electron diffraction analysis, this influences only the parameters concerning the N-N distances and only to such an extent that they remain within the uncertainty of these parameters. The N3 moiety of IN3 is almost linear and is subject to the so-called "shrinkage-effe~t";~ therefore the bond angles should be transformed into ra parameters, which are already free from the effect of perpendicular vibrations. Utilizing the perpendicular amplitudes calculated by the normal coordinate analysis, these ra angles were calculated, and these are the ones to be compared with the calculated equilibrium bond angles. Results and Discussion The results of the structure analysis are summarized in Table 1. There is agreement with most of the computed parameters within experimental error. The two sets of parameters should
Darameter (N-N)mean
A(")
N2-N3 Nl-N2 I-N N-N-N
I-N-N
ED 1.204 f 0.004 0.113 f 0.022 1.147 f 0.013 1.260 f 0.013 2.120 f 0,010 169.6 f 3.0 106.6 f 1.1
caic2,b 1.212 0.091 1.167 1.258 2.133 171.4 110.2
calc4." 1.210 0.090
1.165 1.255 2.120 171.6 110.4
Estimated total errors,' indicated as error limits, include 2/2 times the least-squares standard deviation, a systematic error of 0.2%, and a third error term reflecting the uncertainty due to the presence of possible other, minor components in the vapor. MP2/6-31G*,for I: [5s5pdl]/ [2s2pld](DZ+P)basis set and quasi-relativistic pseudopotential. MP2/ 6-31G(d,p),for I: split (4333211433211431)basis set, augmented by a set of six d-functions. not be the same numerically since they correspond to different representations of the molecular geometry (cf., e g , ref 8). It is noteworthy that the ra parameter for LNNN agrees well with the calculated equilibrium bond angle, while the r, angle for this parameter would be 167.9 & 3.0". There are two parameters for which the agreement between the electron diffraction results and the calculations is poor. One of them is the N-N mean bond length where the computed bond length may be exaggerated as re should be smaller than r,.* The other is the I-N-N bond angle. The value of the X-N-N angle in FN3 is 104" from both experiment9 and calc~lation.~ For the other three halogen azides the X-N-N bond angle is around 109-110". Our experimental value for IN3 is, however, smaller, 106.6 f 1.1". It is difficult to find a reasonable explanation for this discrepancy. However, it is possible that an explanation may be found by taking into account the different bonding types in these molecules. According to ref 4, the partial charges on F and N1 are -0.302e and +0.245e, respectively, in FN3; that is, the polarity of the X-N bond is Xa--N6+. For all the other halogen azides the polarity of this bond reverses to X6+-Nd-, with increasing polarization and less covalent character toward IN3, in which the partial charges on I and N1 are f0.353e and -0.33 le, respectively. The contour diagrams of the Laplacian distribution, V@(r),show large regions of charge depletion in both the F-N and the I-N interatomic region in FN3 and IN3, respectively, while much less so in the other azides. Therefore, we may suppose that fluorine and iodine azides are somewhat similar and at the same time different from the other halogen azides. As to the calculated I-N-N bond angle, if there are any effects of basis set problems, they can be expected with the heaviest iodine atom. Acknowledgment. This research was supported by the Hungarian Scientific Research Foundation (OTKA No. 2103), the Fonds der Chemischen Industrie, the Deutsche Forschungs-
Molecular Structure of Iodine Azide gemeinschaft (KL 636/2-2),the Bundesministerium kBildung und Wissenschaft (Graduiertenkolleg: Synthese und Strukturaufklmng niedennolekularer Verbindungen), and the GermanHungarian (TU Berlin, TU Budapest) partnership.
References and Notes (1) (a) Hungarian Academy of Sciences. (b) Technische Universitiit Berlin. (c) Budapest Technical University. (2) Buzek, P.; KlapBtke, T. M.; Schleyer, P. v. R.; Tomieporth-Oetting, I. C.; White, P. S. Angew. Chem., Int. Ed. Engl. 1993,32, 275.
J. Phys. Chem., Vol. 98, No. 40, 1994 10097 (3) Hargittai, M.;Tomieporth-Oetting,I. C.; Klaptitke, T. M.; Kolonits, M.; Hargittai, I. Angew. Chem., Int. Ed. Engl. 1993,32, 759. (4)Otto, M.; Lotz, S. D.; Frenking, G. Inorg. Chem. 1992,31, 3647. ( 5 ) Christen, D. J . Mol. Struct. 1978,48, 101. (6) See, for example: Hargittai, I. In Stereochemical Applications of Gas-Phase Electron Difiuction; Hargittai, I., Hargittai, M., Eds.; VCH: New York, 1988; Part A, Chapter 7. (7) Hargittai, M.; Hargittai, I. J . Chem. Phys. 1973,59, 2513. (8) Hargittai, M.; Hargittai, I. Int. J . Quantum Chem. 1992.44,1057. (9) Christen, D.; Mack, H. G.; Schatte, G.; Willner, H. J . Am. Chem. SOC. 1988,110, 707.
