CH₃CN: X-ray structural investigation of a unique single crystal. β → α phase transition and crystal structure

The phase transition from the low- (β) to the high-temperature (α) form of acetonitrile, CH₃CN, has been directly observed and studied on a unique single crystal. Both the β and α structures have been determined at temperatures close to the transition temperature (206 K and 201 K), taking advantage of the hysteresis. A single crystal of the β form was obtained for the first time.

Computing details

For both compounds, data collection: CAD-4 EXPRESS (Enraf-Nonius, 1993); cell refinement: CAD-4 EXPRESS (Enraf-Nonius, 1993); data reduction: CADAK (Savariault,1991); program(s) used to solve structure: SHELXS96 (Sheldrick, 1990); program(s) used to refine structure: SHELXL96 (Sheldrick, 1996); molecular graphics: ORTEP III (Burnett & Johnson, 1996); software used to prepare material for publication: SHELXL96 (Sheldrick, 1996).

Figures

(I) acetonitrile

Crystal data

C2H3N	F(000) = 88
Mr = 41.05	Dx = 1.058 Mg m^−3
Orthorhombic, Cmc21	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: C 2c -2	Cell parameters from 25 reflections
a = 6.187 (1) Å	θ = 5.7–20.1°
b = 5.282 (3) Å	µ = 0.07 mm^−1
c = 7.887 (4) Å	T = 206 K
V = 257.7 (2) Å^3	Cylinder, colourless
Z = 4	1.2 × 0.5 × 0.3 × 0.15 (radius) mm

Data collection

Nonius CAD4 diffractometer	99 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.000
Graphite monochromator	θmax = 24.9°, θmin = 5.1°
ω–2θ scans	h = 0→7
136 measured reflections	k = 0→6
136 independent reflections	l = −9→0

Refinement

Refinement on F^2	Hydrogen site location: difference Fourier map
Least-squares matrix: full	All H-atom parameters refined
R[F^2 > 2σ(F^2)] = 0.058	w = 1/[σ^2(Fo^2) + (0.0888P)^2 + 0.1685P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.149	(Δ/σ)max = 0.037
S = 1.20	Δρmax = 0.18 e Å^−3
136 reflections	Δρmin = −0.17 e Å^−3
26 parameters	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.01 (7)
Primary atom site location: structure-invariant direct methods	Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881
Secondary atom site location: difference Fourier map	Absolute structure parameter: −10 (10)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. Weighted R-factors wR and goodnesses of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold_expression of F² > σ(F²) is used only for calculating R_factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
N	0.0000	0.4380 (14)	−0.0974 (18)	0.070 (3)
C1	0.0000	0.0776 (15)	0.1243 (18)	0.053 (2)
C2	0.0000	0.2762 (16)	0.0000	0.053 (2)
H1	0.0000	−0.104 (15)	0.080 (12)	0.07 (3)*
H2	0.149 (11)	0.087 (9)	0.201 (8)	0.083 (19)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N	0.070 (5)	0.069 (6)	0.070 (6)	0.000	0.000	0.014 (4)
C1	0.058 (5)	0.050 (4)	0.052 (5)	0.000	0.000	0.008 (5)
C2	0.035 (3)	0.067 (4)	0.058 (4)	0.000	0.000	−0.011 (5)

Geometric parameters (Å, º)

N—C2	1.149 (12)	C1—H1	1.02 (8)
C1—C2	1.436 (12)	C1—H2	1.11 (7)
C2—C1—H1	117 (6)	H1—C1—H2	103 (4)
C2—C1—H2	110 (3)	N—C2—C1	178.9 (12)

(II) acetonitrile

Crystal data

C2H3N	F(000) = 88
Mr = 41.05	Dx = 1.028 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 25 reflections
a = 4.102 (3) Å	θ = 5.2–21.5°
b = 8.244 (7) Å	µ = 0.07 mm^−1
c = 7.970 (7) Å	T = 201 K
β = 100.1 (1)°	Cylinder, colourless
V = 265.3 (4) Å^3	1.2 × 0.5 × 0.3 × 0.15 (radius) mm
Z = 4

Data collection

Nonius CAD4 diffractometer	202 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.057
Graphite monochromator	θmax = 21.9°, θmin = 3.6°
ω–2θ scans	h = 0→4
376 measured reflections	k = 0→8
324 independent reflections	l = −8→8

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.047	All H-atom parameters refined
wR(F^2) = 0.111	w = 1/[σ^2(Fo^2) + (0.036P)^2 + 0.1879P] where P = (Fo^2 + 2Fc^2)/3
S = 1.14	(Δ/σ)max < 0.001
324 reflections	Δρmax = 0.17 e Å^−3
41 parameters	Δρmin = −0.17 e Å^−3
0 restraints	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.07 (4)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. Weighted R-factors wR and goodnesses of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold_expression of F² > σ(F²) is used only for calculating R_factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
N	0.4547 (9)	0.2657 (5)	0.4613 (4)	0.0710 (15)
C1	0.0949 (12)	0.4579 (6)	0.2478 (6)	0.0586 (14)
C2	0.2946 (9)	0.3498 (5)	0.3672 (5)	0.0507 (13)
H1	−0.108 (11)	0.402 (5)	0.166 (5)	0.089 (14)*
H2	0.233 (11)	0.518 (6)	0.186 (6)	0.113 (18)*
H3	−0.050 (11)	0.538 (6)	0.301 (5)	0.103 (16)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N	0.076 (3)	0.070 (3)	0.064 (2)	0.005 (2)	0.0028 (18)	0.008 (2)
C1	0.058 (3)	0.060 (3)	0.055 (3)	0.005 (2)	0.001 (2)	0.008 (2)
C2	0.056 (2)	0.050 (3)	0.047 (2)	−0.007 (2)	0.0101 (19)	−0.009 (2)

Geometric parameters (Å, º)

N—C2	1.141 (5)	C1—H2	0.96 (5)
C1—C2	1.448 (6)	C1—H3	1.03 (5)
C1—H1	1.07 (5)
C2—C1—H1	115 (2)	H1—C1—H3	95 (3)
C2—C1—H2	110 (3)	H2—C1—H3	108 (4)
H1—C1—H2	112 (3)	N—C2—C1	179.3 (4)
C2—C1—H3	115 (2)