2,6-Di­methyl­pyrazine at 5 K: a neutron-diffraction study

Single-crystal neutron-diffraction techniques are used to determine the crystal structure of 2,6-di­methyl­pyrazine (DMP), C₆H₈N₂, at 5 K. The space group is P2₁/a with Z = 4, as at room temperature. The methyl groups are ordered. There are two crystallographically inequivalent methyl groups in the unit cell. Different rotational dynamics may account for the two rotational tunnelling transitions observed with inelastic neutron-scattering techniques.

Key indicators

• Single-crystal neutron study
• T = 5 K
• Mean [sigma](C-C) = 0.002 Å
• R factor = 0.032
• wR factor = 0.016
• Data-to-parameter ratio = 7.9

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level B:



DIFMN_02  Alert B The minimum difference density is < -0.1*ZMAX*1.00
          _refine_diff_density_min given =     -0.730
          Test value =     -0.700

Author response: ...The corresponding peak F1 has been located: C3-F1=0.65\%A, C4-F1=1.545\%A, C6-F1=1.401 \%A. this is nonsensical

DIFMX_01  Alert B The maximum difference density is > 0.1*ZMAX*1.00
          _refine_diff_density_max given =      0.780
          Test value =      0.700

Author response: ...The corresponding peak F2 has been located: H51-F2=1.009\%A and is external to the ring. this is nonsensical

REFNR_01  Alert B Ratio of reflections to parameters is < 8 for a
          centrosymmetric structure
          sine(theta)/lambda                0.6904
          Proportion of unique data used    0.7184
          Ratio reflections to parameters   7.8973

Author response: ...We have performed only a test at 5K in order to detect a possible phase transition between 20 and 5K. We found no evidence of phase transition . The structure at 5K is very similar to that at 20K and the R value is satisfactory.

 Alert Level C:



CELLK_01  Alert C Check that the cell measurement temperature is in Kelvin.
          Value of measurement temperature given =     5.000

DIFMN_03  Alert C The minimum difference density is < -0.1*ZMAX*0.75
          The relevant atom site should be identified.

DIFMX_02  Alert C The minimum difference density is > 0.1*ZMAX*0.75
          The relevant atom site should be identified.

General Notes



ABSMU_01  Radiation type not identified. Calculation of
          _exptl_absorpt_correction_mu not performed.


 0 Alert Level A = Potentially serious problem

 3 Alert Level B = Potential problem

 3 Alert Level C = Please check

---

S1. Comment

As found for the structure of this material, (I), at 20 K (Kaiser-Morris et al., 2001), the space group is P2₁/a (monoclinic) with four molecules per unit cell. There is no evidence for any phase transition between 20 and 5 K, and no significant changes of the lattice parameters below 20 K.

The structure consists of parallel layers of planar molecules perpendicular to the (201) plane (Kaiser-Morris et al., 2001). The protons of the methyl groups are quite localized at all temperatures. For each methyl group, one of the protons is almost in the molecular plane. The displacement ellipsoids for both methyl groups correspond quite well to those anticipated for hindered rotors with a rather high potential barrier and threefold symmetry (Fig. 1). There are two different crystallographic environments for the methyl groups linked to the same pyrazine ring. The different local potentials may account for the different tunnelling frequencies. This is confirmed by further inelastic neutron scattering measurements performed on single crystals (Nicolaï et al., 1998).

S2. Experimental

2,6-Dimethylpyrazine (DMP) is hygroscopic and melts at 311 K. We performed neutron-diffraction experiments with a single-crystal at 5 K on the four-circle neutron diffractometer 5 C2 at the LLB (Saclay, France). A large single-crystal (1 × 1 × 5 cm) was obtained at low temperature. A small single-crystal (5 × 5 × 5 mm) was cut, glued on a goniometer head and oriented on 5 C2. The measurements were performed with the ω scan mode and an incident wave length close to 0.83 Å selected with the Cu (220) monochromator.

Computing details

Data collection: DIF4N (modified Linux version of DIF4; Stoe & Cie; 2000); cell refinement: DIF4N; data reduction: PRON (modified version of REDU4; Stoe & Cie, 2000); program(s) used to refine structure: CRYSTALS (Watkin, Prout, Carruthers & Betteridge, 1996); molecular graphics: CAMERON (Watkin, Prout & Pearce, 1996); software used to prepare material for publication: CRYSTALS.

Figures

Fig. 1. The molecular structure of (I) at 5 K, with 50% probability displacement ellipsoids. For a packing diagram, see the preceding paper (Kaiser-Morris et al., 2001).

2,6 dimethylpyrazine

Crystal data

C6H8N2	Dx = 1.23 Mg m^−3
Mr = 108.14	Melting point: not measured K
Monoclinic, P21/a	Neutron radiation, λ = 0.8308 Å
a = 7.287 (7) Å	Cell parameters from 16 reflections
b = 10.725 (9) Å	θ = 9.8–21.5°
c = 7.452 (8) Å	µ = 0.08 mm^−1
β = 90.37 (9)°	T = 5 K
V = 582.4 Å^3	Prism, white
Z = 4	5.0 × 5.0 × 5.0 mm
F(000) = 114.73

Data collection

Orphée reactor (Saclay, France): 5C2 four-circle diffractometer	Rint = 0.022
Radiation source: Orphée reactor Saclay France	θmax = 35°, θmin = 1°
Cu (220) monochromator	h = −10→10
ω scans	k = −14→4
2026 measured reflections	l = −10→4
1605 independent reflections	2 standard reflections every 450 min
1153 reflections with I > 3σ(I)	intensity decay: 0.0%

Refinement

Refinement on F	All H-atom parameters refined
Least-squares matrix: full	Chebychev polynomial with 5 parameters: 0.951, -3.12, 0.00654, -0.885, -0.649 (Carruthers & Watkin, 1979)
R[F^2 > 2σ(F^2)] = 0.032	(Δ/σ)max = 0.0002
wR(F^2) = 0.016	Δρmax = 0.78 e Å^−3
S = 1.04	Δρmin = −0.73 e Å^−3
1153 reflections	Extinction correction: Larson (1970)
146 parameters	Extinction coefficient: 2.39 (18)

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

	x	y	z	Uiso*/Ueq
N1	0.12244 (9)	0.98280 (6)	0.23650 (9)	0.0077
N2	0.19686 (9)	0.74311 (6)	0.36758 (8)	0.0067
C1	0.19508 (13)	0.96467 (8)	0.39921 (12)	0.0070
C2	0.23301 (13)	0.84503 (8)	0.46568 (12)	0.0058
C3	0.12436 (13)	0.76015 (8)	0.20379 (12)	0.0057
C4	0.08834 (13)	0.88017 (8)	0.13934 (13)	0.0070
C5	0.31588 (14)	0.82873 (9)	0.64883 (12)	0.0094
C6	0.08275 (14)	0.64663 (9)	0.09368 (12)	0.0089
H11	0.2236 (4)	1.0470 (2)	0.4804 (3)	0.0234
H41	0.0295 (3)	0.8937 (2)	0.0054 (3)	0.0217
H51	0.3166 (6)	0.7316 (3)	0.6874 (4)	0.0439
H52	0.4577 (4)	0.8614 (4)	0.6497 (4)	0.0484
H53	0.2416 (5)	0.8827 (4)	0.7475 (4)	0.0447
H61	0.0098 (6)	0.6710 (3)	−0.0284 (4)	0.0420
H62	0.2071 (4)	0.5992 (3)	0.0547 (5)	0.0449
H63	−0.0003 (5)	0.5821 (3)	0.1693 (4)	0.0367

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0115 (3)	0.0048 (3)	0.0068 (3)	0.0008 (2)	−0.0009 (2)	0.0009 (2)
N2	0.0095 (3)	0.0043 (3)	0.0062 (3)	−0.0002 (2)	−0.0008 (2)	−0.0003 (2)
C1	0.0105 (4)	0.0037 (4)	0.0068 (4)	0.0002 (3)	−0.0008 (3)	−0.0002 (3)
C2	0.0098 (4)	0.0039 (3)	0.0036 (3)	0.0000 (3)	−0.0006 (3)	−0.0004 (3)
C3	0.0071 (4)	0.0051 (3)	0.0051 (4)	0.0000 (3)	−0.0000 (3)	−0.0000 (3)
C4	0.0104 (4)	0.0052 (4)	0.0055 (4)	−0.0001 (3)	−0.0010 (3)	0.0006 (3)
C5	0.0137 (4)	0.0084 (4)	0.0059 (4)	0.0001 (3)	−0.0022 (3)	−0.0006 (3)
C6	0.0128 (4)	0.0068 (4)	0.0071 (4)	−0.0007 (3)	−0.0011 (3)	−0.0017 (3)
H11	0.0364 (12)	0.0124 (8)	0.021 (1)	0.0004 (8)	−0.0038 (9)	−0.0034 (7)
H41	0.0304 (11)	0.0195 (9)	0.0152 (9)	0.0004 (8)	−0.0078 (8)	0.0022 (8)
H51	0.079 (2)	0.018 (1)	0.0341 (14)	−0.0036 (13)	−0.0250 (16)	0.007 (1)
H52	0.0269 (12)	0.084 (3)	0.0342 (13)	−0.0246 (15)	−0.0110 (11)	0.0167 (15)
H53	0.0570 (17)	0.0588 (19)	0.018 (1)	0.0343 (17)	−0.0023 (11)	−0.0116 (12)
H61	0.075 (2)	0.0251 (13)	0.0257 (12)	0.0038 (13)	−0.0272 (14)	−0.002 (1)
H62	0.0262 (12)	0.0372 (14)	0.071 (2)	0.0044 (11)	0.0037 (12)	−0.0332 (15)
H63	0.0532 (17)	0.0244 (12)	0.0327 (13)	−0.0212 (11)	0.0139 (12)	−0.0052 (9)

Geometric parameters (Å, º)

N1—C1	1.3341 (11)	C3—C6	1.4980 (12)
N1—C4	1.3397 (11)	C4—H41	1.093 (2)
N2—C2	1.340 (1)	C5—H51	1.081 (3)
N2—C3	1.3394 (11)	C5—H52	1.092 (3)
C1—C2	1.4023 (12)	C5—H53	1.083 (3)
C1—H11	1.090 (2)	C6—H61	1.083 (3)
C2—C5	1.4991 (13)	C6—H62	1.081 (3)
C3—C4	1.3982 (13)	C6—H63	1.080 (3)
C1—N1—C4	116.27 (7)	C3—C4—H41	120.48 (15)
C2—N2—C3	117.44 (7)	C2—C5—H51	110.82 (18)
N1—C1—C2	122.04 (8)	C2—C5—H52	110.11 (17)
N1—C1—H11	117.35 (15)	H51—C5—H52	107.8 (4)
C2—C1—H11	120.61 (15)	C2—C5—H53	110.83 (17)
N2—C2—C1	121.07 (8)	H51—C5—H53	109.6 (3)
N2—C2—C5	118.56 (8)	H52—C5—H53	107.6 (3)
C1—C2—C5	120.37 (8)	C3—C6—H61	111.12 (17)
N2—C3—C4	120.71 (8)	C3—C6—H62	111.25 (17)
N2—C3—C6	117.74 (8)	H61—C6—H62	107.2 (3)
C4—C3—C6	121.55 (8)	C3—C6—H63	110.30 (17)
N1—C4—C3	122.47 (8)	H61—C6—H63	108.6 (3)
N1—C4—H41	117.04 (15)	H62—C6—H63	108.2 (3)