J. Phys. Chem. 1992, 96, 7976-7978
7976
Conformational Analysis. 17. Reinvestigation of the Structure and Corrformatlon of Tetrabromoformaldazlne (1,I,4,4-Tetrabrom~2,3-dlaza-l,3-butadiene)by QabPhase Electron Dlffractlon: Dynamics of Torsion about the N-N Bond Kolbjerm Hagent and Kenneth Hedberg* Department of Chemistry, Oregon State University, Coruallis, Oregon 97331 -4003 (Received: April 21, 1992)
The structure and conformation of tetrabromoformaldazine(1,1,4,4-tetrabroma-2,3dm-1,3-butadiene, Br20N-N-CBr2) has been reinvestigated with use of gas-phase electron-diffraction data obtained in our earlier investigation. The work was stimulated by an investigation of the corresponding tetrachloro compound (reported in the following article) in which a model incorporating large amplitude motion around the N-N bond was required. A similar model for tetrabromoformaldazine that also included corrections for vibrational averaging gave better agreement with experiment than was obtained before. The torsional potential (V(T) = Vo(1- ~ ( T / T ~+) (T/T~)') ~ has a small maximum at the planar anti position (Vo = 1.4 (12) kcal/mol) and minima at f59 (9)' away from anti. The dynamic model led to some changes in the values of some of the bond lengths and bond angles. The new results for the principal distances ( r ) and angles (fa) are r(C=N) = 1.274 (10) A, r(N-N) = 1.381 (23) A, r(C-Br) = 1.880 (5) & LN-N=C = 117.4 (27)'fLN==C-Br5 = 124.0 (19)O, and LN==C-Br6 = 120.4 (19)'.
Introduction
Some years ago we investigated the molecular structures of formaldazine' (2,3diaza-l ,fbutadiene, H2C=N-NICH2) and tetrabromoformaldazincZ (Br2C=N-N=CBr2) by gas-phase electron diffraction. For formaldazine the major conformer was found to be planar anti and the minor conformer a nonplanar gauche, but for tetrabromoformaldazine only a nonplanar skew conformer was observed. We have recently investigated tetrachloroformaldazinc? (Cl2C=N-NPCCl2) by gas-phase electron diffraction and ab initio calculations. For this molecule the theoretical results suggested a double minimum in the torsional potential with a low barrier at the anti position, a circumstance better represented by a dynamic model (i.e., a dynamic representation of torsional motion around the N-N bond) than by the usual model consisting of a single conformer. The dynamic model was based on the torsional potential function V(T)= Vo(1 - 2 ( 7 / ~ ~ ) ~( T) / T ~ ) ~where ), Vois the height of the potential at the planar anti position, f0is the torsion angle for the potential minimum, and T~ = 0' for the anti form. Such a dynamic model was not tested for tetrabromoformaldazine in our earlier investigation2 with the result that a moderate twist about the C-N bond was required in order to fit the experimentaldata. In view of our new results for tetrachloroformaldazine,it seemed advisable to test a similar dynamic model for the bromo compound as well. This is a short report of our results.
YI
n
F
+
Experimental Section
The sample preparation, the conditions of the diffraction experiment, and treatment of the data are given in the earlier publication.* The original data were used without change. Structure Analysis and Final Results
The geometry of tetrabromoformaldazine (Figure 1) was described by the six parameters r(C-N), r(N-N), r(C-Br), LN-N-C, LN=C-BrS, and LN=C-Br,. Torsion around the N-N bond (#(CNNC) = 180 - T) was represented by introduction of pseudoconformers at # = +180°, f165O, +150°, +135', f120', flOS', and f90'. Each pseudoconformer was given weight according to P a exp(-V(T)/RT)where V(T) = Vo(l
- ~ ( T / T ~+) (~T / T ~ ) ~ ) . 'On leave from University of Trondheim, Norway.
0022-3654/92/2096-7976$.03.00/0
Figure 1. Diagram of tetrabromoformaldazine with atom numbering. Torsion angle Q = 121O .
COITCC~~OM of interatomic distancGp for the effects of vibrational averaging can be calculated from an appropriate quadratic force field. No spectroscopic information has been reported for tetrabromoformaldazine,and the computational time required for this molecule with our facilities prevented an ab initio calculation similar to that carried out for the tetrachloro compound. We adopted the force field for C2N2C14,3modified for the bromine substitution from comparisons of force fields for related bromo and chloro compounds. This force field was then used to calculate approximate values for centrifugal distortion, 6r, perpendicular amplitude corrections, K,and raot mean square vibrational amplitudes, I, using the program ASYM20.4 Refmements of the structure were made by the method of least squares,5adjusting a theoretical sZ,,,(s) curve simultaneously to the two sets of experimental data, one from each camera distance. Calculations of theoretical radial distribution (RD)curves quickly showed that a good fit of experimental and theoreticalCUNCP could be obtained with use of the dynamic model. In the final refinements all six geometrical parameters, the two parameters determining the N-N torsional potential, and eight amplitude parameters were refined simultaneously. The results from this refinement are given in Table I. Table I1 is the comlation matrix for this model, and the intensity curve for the final model is shown 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 1917
Structure and Conformation of Br2C=N-N=CBr2 TABLE I: Structural Panmeten for Tetrabromfomuhzi& ra/La
E'
1.266 (10) 1.377 (23j 1.871 ( 5 ) 117.4 (27) 124.0 (19) 120.4 (19) 1.4 (12) 121 (9) 0.130
1.274 1.382 1.880
r, 1.272 1.380 1.878
C2N2Br 4
I 10.0411 i0.048j 0.068 (8)
P
EXPERIMENTRL LONG CRNERR
NIOOLE CAMERA
ices (Torsion Dependent Distances Are for 4 = 120' ( T = 60')) 2.228 (22) 2.233 2.231 (0.0601 2.794 (27) 2.801 2.796 0.113 2.732 (25) 2.747 2.742 0.114) (22) 3.161 ( 5 ) 3.170 3.168 0.092 ( 5 ) 3.149 (40) 3.154 3.151 [0.088] 3.821 (66) 3.827 3.823 0.120 }(lo3, 4.781 (44) 4.789 4.787 0.090 4.921 (74) 4.927 4.920 0.196 (149) 4.904 (64) 4.913 4.897 0.283 (200) 6.535 (50) 6.540 6.535 0.106 (40) 2.929 (60) 2.936 2.931 0.128 3.994 (21) 4.005 4.003 0.088
n
"Distances (r) and amplitudes (I) are in Angstroms; angles ( L ) are in degrees. Parenthesized values are 2a and include estimates of uncertainty in voltage/nozzle heights and of correlation in the experimental data. Values in square brackets were kept constant in the least squares refinements; those in braces were refined as a group. Vo is the barrier height in kilocalories per mole at the planar anti position. c$o is the angle where the torsional potential has a minimum. 4 = 180° at the planar anti position. d R = [ ~ ~ ~ A ~ / w ~ ( s ~ l ~ ( o b swhere d ) ) ~Ai] ~=/ ~ sili(obsd)- sili(calcd).
RVERRGE CURVES
v I\
- -
-
THEORETICRL
- ----
in Figure 2, together with experimental and difference curves. Figure 3 shows experimental and difference RD curves.
A-
-
DIFFERENCE
A
I
I
10
20
h
..--
s
Discussion
Figure 2, Intensity curves for tetrabromoformaldazine. The PI, experimental curves are shown magnified 5 times with respect to the backgrounds on which they are superimposed. The average curves are ~($1, - background). The theoretical curve is calculated from parameter values given in Table I.
Use of a dynamic model to describe the structure of tetrabromoformaldazine shows clearly that a good fit to the experimental data may be obtained without introduction of a twist about the N-C double bonds. (Tests of such a twist parameter led to insignificant improvement in the fit.) In view of the theoretical results for other formaldazines,it secms likely that the dynamic model is a more realistic representation of the structure of tetrabromoformaldazine than the singlaconformer model invoked for our earlier study. The example of this molecule should Serve as a reminder that model assumptions frequently determine the final picture of the structure. It also strongly indicates that
dynamic models should be used in the analysis of electron-diffraction data whenever large amplitude motion is likely to be present. Our l a s t squares results showed that the height of the potential at the anti position cannot be measured accurately. However, they do indicate strongly that the potential hump at 4 = 180° (the planar anti conformation) is significantly higher in C2N2Br4 than in C2N2Ctwhen a barrier height of only about 0.1 kcal/mol was found.3 If we assume that this hump is a result of steric repulsion between the halogen atoms and the unshared electron pairs cis to them on the nitrogen atoms? then this difference is
TABLE 11: Correlation Matrix (XlOO) for Pmwters of Tetrabrmfoddnzine param %Sa rl r2 r3 f4 L5 L6 VO 70 r(C=N) r(N-N) r(C-Br) fN-N=C LN=C-Br, LN-C-Br,
VO 70
I(C-Br) I(NZ*..Br,) I(Br5-.-Br6) I(CI..-Br,) I(Br,. .Br7) I( * I(Br6---Br8) I(N2.* SBr,)
0.335 0.818 0.17 1 96.4 68.8 68.7 44.1 308 0.263 0.751 0.138 3.67 5.26 7-08 1.42 0.657
100
44 100
-26 -18 100
-52 -74 16 100
26 29 18 -32 100
-28 -31 3 32 -97 100
-11 -11 9 28 -50
51 100
-15 -24 16 57 -17 17 -16 100
19
10 25 -11 -22 -13 13 -6 2 100
110
5
16 -17 -30 -66 64 16 -6 32 100
Ill
112
113
114
/I5
I16
11 25 -2 -17
1 10 -7 -18 -21 21
-8 -14 -8 6 -15 13 16 -25 -2 3 - 6 26
