Conformation and molecular structure of 1-chlorobutane as determined by gas-phase electron diffraction and molecular mechanics calculations

Journal of Molecular Structure, 96 (1983) 339-346 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

CONFORMATION AND MOLECULAR STRUCTURE OF 1CHLOROBUTANE AS DETERMINED BY GAS-PHASE ELECTRON DIFFRACTION AND MOLECULAR MECHANICS CALCULATIONS

STEINAR

FAGERLAND,

TERJE

Department of Chemistry, Trondheim (Norway)

RYDLAND

and REIDAR STGLEVIK

University of Trondheim,

NLHT, Rosenborg,

N-7000

RAGNHILD SEIP Department

of Chemistry,

University of Oslo, Blindern, Oslo 3 (Norway)

(Received 25 May 1982)

ABSTRACT Gaseous l-chlorobutane has been studied at 18’C by electron diffraction. The conformational composition obtained comprises 50% GA, 25% AA, 15% GG, 5% AG and 5% GG” with estimated error limits of about f 5%. Normal values of the structural parameters have been determined and low-frequency torsional vibrations have been compared with observations from vibrational spectroscopy. Results from molecular mechanics calculations largely agree with the electron diffraction data. The results are compared with those of an earlier electron diffraction investigation.

INTRODUCTION

1-Chlorobutane (CLB) exhibits five possible conformers, GA, AA, GG, AG and GG”, where the first and second symbols respectively refer to the Cl-C2 and C2-C3 axes. Thus, the G in the conformer GA indicates that the halogen (X) is gauche in the sequence of atoms X-C1-C2-C3, while A indicates that the methyl group is anti in the sequence CI--C2-C3-Me as used in Table 1. The conformers GA, GG, AG and GG” exist as enantiomeric pairs, whereas AA does not. In an early gas-phase electron diffraction study it was concluded that CLB [l, 21 had a conformer distribution of 37% GA, 24% GG”, 17% GG, 11% AA and 11% AG at room temperature with 24% GA, 0% GG”, 16% GG, 36% AA and 24% AG for 1-bromobutane [ 31. These data have frequently been referred to since, in particular the high percentage of GG”, the CLB conformer with the chlorine and the methyl group on the same side of the C1C2C3skeleton. For l-bromobutane no contribution from GG” wasdetected

[31-

Several vibrational spectroscopic studies of halobutanes have been carried out and a recent report [ 41 reviews all previous works. The conclusion of 0022-2860/83/0000-0000/$03.00

0 1983 Elsevier Scientific Publishing Company

340

TABLE 1 Results of molecular mechanics calculations of the conformational parameters for 1-chlorobutane, X=Cl Conformer

GA

AA

GG

energies and structural

AG

GG"

-& I

I

I

I

x-c,-c,--$-c,-

I

I

I

I

I

I

-C-C-C-C-

I

I

I

I

I

I

I

x-c-c-c-cl

I

I

I

I

I

I

I

I

I

X

I

X

I

I

I

I

“-v~-

-C-C-C-

I

I

I

I

I I

-c-c-cx

-C-

I (kJ moZ--) With chargesa Energy

Without charges

Ob Ob

Torsionaangles(@I)' ("1 @Cl--2) +113.2 N2-3) -1.0 @(34) 0 Bond anglesa(", LCCC LCCX

112.8 110.8

2.6 2.1

1.8 1.5

0 0 0

112.2 109.7

aq(H,) = +0.042, q(C,) = +0.003, q(X) = -0.152, -0.013 e. bEnergies relative to that of GA. CExactly staggered when 4 is 0 or 120”.

6.3 5.4

4.1 4.2

+119.7 -118.8 -4.7

-1.8 +111.7 +3.5

+99.0 f112.3 +8.0

114.8 111.1

113.8 109.7

115.2 111.4

q(H,, H,, H,) = +O.Oll, q(C,,

C,, C!,) =

ref. 4 is that the conformer GG” is not identified in the gas state nor in the liquid or glassy state of CLB. It is known that CLB exists in the AA form in the solid state [ 51. Thus the results of ED and vibrational spectroscopy in the gas phase do not agree. However, it was discovered that the original ED work [2] probably involved an error in the computation of the internuclear distances. Using the bond lengths and bond angles from ref. 2, the length of the Cl-C3 (anti) distance is found to be 4.15 A, not 4.711 A (trans) as given in Table 1 of ref. 2. It is also obvious now that the value of 0.115 A for the mean amplitude of vibration (u) for the Cl-C, distance in GG” is too small (Table 2), as is also the case for Cl--C3 (gauche). In a conformational study of a compound with five conformers it is essential that the u values are known and included as fixed parameters in the analysis. Moreover, that part of the RD curve where the C1-Cq distance (2.7 A) of the GG” conformer occurs is a complex peak containing contributions from all other conformers. CALCULATIONS

Conformational

energies

and torsional

barriers

Molecular mechanics calculations were carried out using non-bonded potential functions in the Morse formulation [ 61. The potential parameters

I

-c/

341 l’AI3I.S 2 Mean amplitudes of vibration, anti and g = gauche

u = (A&,

Contact

r(A)

ux

3-c 2-X Z-H >.,.X >...H >...C

1.533 1.795 1.110 2.74 2.16-2.19 2.54 2.39 3.16-3.27 4.14 2.74-2.81 3.50-3.51 3.91 3.04-3.08 3.71 2.85-2.92 4.63 5.28 3.98 4.70 2.70 4.18-4.23 4.72

51-52 53 78-79 73 107-109 71-72 109 144-146 71-72 157-173 105-106 74 136-166 104 164-170 173 96 293 162 234 162-171 121

!I...X QC( g) K.*C(a) -r.C(g) i**C(a) ‘.*C(a) _j ?.C(g) <.*.H(a) <...H(g) <*,C(ga) <..C(aa) (C(gg) <“C(ag)
103a

for 1-chlorobutane

at 18°C. X = chlorine, a =

Contact

r(A)

u x 10’ .&

H...C(ag) H-C(gg”) H.*.C(gg) X...H(gg) X..H(gg”) X...H(ag) X...H(ga) X...H(aa) X...H(gaa) X...H(gag) X...H(gag”) X...H(aaa) X...H(aag) X...H(ggg) X...H(gg”) X...H(gga) X..+H(aga) X...H(agg) X...H(agg”) X...H( gga) X...H(ggg) X...H(ggg”)

4.04-4.08 2.69-2.74 3.47 3.55-3.69 2.75-2.82 4.44 4.16-4.17 4.91 5.27 5.29 4.73 6.24 5.41 4.70 3.56 4.74 5.76 4.82 4.35 3.65 2.88 1.91

159-166 223-236 261 244-258 232-240 165-166 168-172 123 230 175 296 118 220 300 348 274 170 322 277 278 386 333

-

‘orthe non-bonded interactions He--H, H.-C, H**Cl and CC1 are from ref. 7. ?or C**C values of R. = 3.55 A, R, = 3.98A and E = 0.347 kJ mall’ were tsed [lo] . The electrostatic terms were calculated as suggested by Sanderson 8,9] but reduced [lo] by a factor of 1.6. The force constants from ref. 10 vere used for bond lengths and bond angles. Calculated results are given in ‘able 1. The energy values were obtained by simultaneously adjusting the bond lengths, bond angles and torsion angles. It was assumed that the various groups of atoms possess the following ymmetry: C, for XH#-C, CaVfor C-CH, and Czh for CH$-CH& when he four C atoms are coplanar (anti). Within any one conformer all C-H and J-C bonds are equal and the CCH angles are equivalent. The torsional angles re@(l-2) forrotationaround the axis C1--C2, $(2-3) for C2-C3 and @(3-4) or C3-C4. Each conformer corresponded to a well defined minimum of the lotential energy function. Low-energy barriers were obtained for the followng transitions (kJ mol-‘): 11.3 for GG + GA, 16.7 for GA + AA and 24.4 or the transition between the enantiomeric forms of GA. Conformational differences between the values of angles CCH and HCH Iere small and all conformational differences between the bond lengths were .egligible [lo] .

342

Conformer AA is exactly 1:2 staggered, corresponding to C, symmetry, while the others deviate somewhat from the staggered form. The most dramatic deviation is found for GG”, where the torsion angle @(l-2) is 21” less than the value of 120” which corresponds to a staggered arrangement of the atoms XH+-C!H2C. Energies for conformers with all torsional angles fixed on staggered values were unreasonably high for GG, AG and GG”. According to the energy values of Table 1 the conformational composition at 18” C is approximately 50% GA, 10% AA, 25% GG, 5% AG and 10% GG”. Equal values for the vibrational and rotational partition functions were assumed in the computations. Vibrational

quantities

A normal coordinate calculation [ 111 was carried out for each conformer and the mean amplitudes of vibration (U and K) were computed [12]. Internuclear distances and their u values are given in Table 2. Complete sets of u and K values for each conformer are given in ref. 13. The force constants, excluding the torsional part, were taken from ref. 14 and the actual values employed are given in ref. 13. The torsional force constants were computed [15] as a sum of six gauche (g) contributions. For the C2--CJ fragments of GG, AG and GG” a contribution for C.-C (gauche) is required in addition to the values of ref. 15. From molecular mechanics calculations a value of Fp* (C*C) = 0.10 mdyn .& radm2was estimated. Non-diagonal elements in the torsional part of the force field were ignored. In Table 3 the calculated values of the torsional frequencies and force constants are given. For each conformer there are three torsions; the CH, mode always having the highest value. According to the normal coordinate treatment the torsional modes are highly coupled therefore the assignments indicated in Table 3 are only approximate. There are also a number of lowfrequency deformation modes (XCC) in CLB. The lower limits of such frequencies are 294,176,246,272 and 295 cm-’ for GA, AA, GG, AG and GG” respectively. The Raman and IR spectra of CLB have been measured previously [4] . Only for AA has a definite assignment of torsional frequencies been suggested. Thus for crystalline AA the CH3 torsional frequency is 237 cm-‘, in good agreement with our calculated value. The remaining torsional frequencies for AA to be identified with two of the reported [4] values are 100, 124,133 and 148 cm-‘. Our calculated values are 101 and 123 cm-‘. The lowest observation assigned to a deformation mode in AA is 166 cm-’ compared to our value of 176 cm-‘. For gas and liquid states the observations [4] below 200 cm-’ are: 120(g), 125(l), 154(g) and 161(l) cm-‘. The values at 124(GA) and 159 cm-’ (GG) from Table 3 may be correlated with these observations. No observations are reported below 101 cm-‘. Thus torsional modes corresponding to values in the region 82-101 cm-’ have not been detected. Additional observations must be obtained in the low-frequency

343 TABLE 3 Torsional frequencies (cm-‘) parentheses (mdyn A radl) Conformer

GA AA GG AG GG”

for 1-chlorobutane

with torsional force constants given in

Fragment CIH,C-CH,C

CH,C-CH,C

CH,C-CH,

124(0.198) 123(0.124) 159(0.198) 108(0.124) 107(0.198)

85(0.094) lOl( 0.094) 82(0.173) 92(0.090) 99(0.183)

224( 0.086) 231(0.086) 220(0.086) 192(0.086) 212(0.086)

region. The difficulties relating to spectral observations below 100 cm-’ have been discussed in a previous paper [ 161 concerned with halopropanes where ED and spectral data were combined. EXPERIMENTAL

AND DATA REDUCTION

The sample of CLB used for ED measurements had a purity in excess of 98%. Data were obtained using Balzers apparatus [17,18] at a nozzle temperature of 18°C. Two sets of plates were obtained Set 1

No. of plates Nozzle-to-plate distance (mm) Electron wavelength, h (a ) Data-range s (A-‘)

3 500.1 0.05862 2.0-15.25

Set 2 5 250.1 0.05862 8.0-29.00

The electron wavelength was determined by calibration against benzene [ 19 ] . The data were treated in the usual way [20] to yield an intensity curve for each plate. Average curves for each set of distances were constructed. A composite curve was then made by connecting the two average curves after scaling. Scattering factors were calculated by the partial-wave method [ 211 using Hartree-Fock atomic potentials [ 223 for C and Cl and a molecular bonded potential for H [23] . Normally the structural parameters and composition are adjusted by leastsquares refinement [20] of the intensities. In the present case, with five conformers under consideration, the adjustments were made by analysis of the rreas and position of the peaks on the RD curve [20]. The final RD curves rre shown in Fig. 1. XESULTS AND DISCUSSION

RD curves for the conformers are shown together with the experimental :urve in Fig. 2. The conformational curves were individually scaled to the

344

a

h Al

0

1

2

CIHzC-CH2-CH2-CH3

3

L

5

3

r(A)

Fig. 1. Experimental (E) and theoretical (T) radial distribution D = E - T. The damping constant was 0.002 A. Fig. 2. Radial distribution curves for the five conformers experimental (E) curve in the conformationally sensitive was 0.002 A.

curves

L

5 r(A)

for l-chlorobutane;

of 1-chlorobutane and the region. The damping constant

experimental one. Only the region showing conformational variation is illustrated. Several features are apparent from a study of these curves. Thus GA and AA are clearly present at a concentration ratio of approximately 1:2. All peaks (a-f) on the experimental curve contain contributions from GA, while AA contributes predominantly to a, d and f. In order to determine the concentrations quantitatively a detailed analysis of the areas and positions of the peaks on the RD curve was carried out. Firstly a theoretical RD curve was calculated for GA using structural parameters from ref. 1, u values from Table 2 and torsion angles from Table 1. Except for the Cl-C bond length, these parameters gave a resonable fit to the experimental curve. A Cl-C length of 1.780 A [1] was clearly too short. In the C1H2C -CH& group the length of the Cl-C bond is normally in the range 1.79-1.80 a. Changing this length from 1.780 to 1.795 a gave a significantly better fit to the Cl-C peak. RD curves of the remaining conformers were then calculated using torsion angles and u values from Tables 1 and 2, respectively. For values of the internuclear distance (r) less than about 2.5 a, the theoretical curves are almost identical and equivalent to the average curve (T) in Fig. 1. Further details concerning the adjustments are given in ref. 13.

345

The final RD curves were calculated with r(C-H) = 1.115, r(C-C) = 1.535, ~(c1-C) = 1.795 a, LCCCl = 110.8, LCCH = 110.6” and LHCH = 109.5” common to all conformers. Values from Table 1 were used for LCCC and the torsion angles, except for GA for which the values LCCC = 112.1 and ~(1-2) = 111” were obtained after adjustments. Shrinkage parameters (6 ) were calculated [24] and included in the adjustments. The 6 values (a) for the long internuclear Cl**C, distances were O.O32(GA), O.O30(AA) and O.O22(AG). For C1**Cg (anti) the 6 values were O.O21(AA) and O.O27(AG) 8. Including these 6 values only improved the fit between the theoretical and experimental RD curves to a slight degree. The final conformational composition obtained was 50% (GA), 25% (AA), 15% (GG), 5% (AG) and 5% (GG”) with an error limit of about +5%. The error limit was estimated according to the criterion that a change in composition should be visually detectable on the calculated RD curve. If all conformational differences between rotational and vibrational partition functions are ignored, the following energy differences (in kJ mol-‘) are obtained from the observed composition at 18°C: AE(AA) = E(AA) - E(GA) = 0, AE(GG) = E(GG) - E(GA) = 2.9, AE(AG) = E(AG) -E(GA) = 5.4 and AE(GG”) = E(GG) - E(GA) = 5.4. The ratios between vibrational as well as rotational partition functions were also calculated. Including these ratios in the computation of AE values yields AE(AA) = -6.8, AE(GG) = 3.4, AE(AG) = 5.0 and AE(GG”) = 5.4 kJ mol-‘. Thus the uncertainty in the partition functions amounts to less than 1 kJ mol-‘, which is of the same magnitude as the experimental uncertainty. The following final values are thus presented: AE(AA) = 0 + 1, AE(GG) = 3 f 1, AE(AG) = 5 and AE(GG”) = 5 kJ mol-‘. For the last two values only a lower limit of about 4 kJ mol-’ can be estimated. Conformer GG” may well be of lower energy than AG, as suggested by the molecular mechanics calculations (Table 1). The energies from molecular mechanics calculations are in good agreement with experiment, although the observed stability of AA is not quite reproduced by the calculations. Considering the arguments given above and the results of the present investigation it is concluded that the concentration of GG” is less than about 10% and not 24% as claimed earlier [2]. With error limits of ?5% our results agree with the previous ones [2] for AG (5 and 11%) and GG (15 and 17%). However, the data for GA (50 and 37%) and AA (25 and 11%) do not agree within error limits. Qualitatively, both investigations find GA the predominant conformer at room temperature in the gas phase. ACKNOWLEDGEMENTS

We are grateful to Hans Volden for measuring the intensities and to Pirkko Bakken for assistance with the drawings. Financial support from Norges almenvitenskapelige forskningsrad (NAVF) is acknowledged.

346 REFERENCES 1 2 3 4

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