2,6-Di­methyl­pyrazine at 20 K: a neutron-diffraction study

Single crystal neutron diffraction techniques are used to determine the crystal structure of 2,6-di­methyl­pyrazine (DMP), C₆H₈N₂, at 20 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.

Key indicators

• Single-crystal neutron study
• T = 20 K
• Mean [sigma](C-C) = 0.002 Å
• R factor = 0.031
• wR factor = 0.019
• Data-to-parameter ratio = 9.2

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 =     -0.890
          Test value =     -0.700

Author response: ...The corresponding peak F1 has been located: C5-F1=0.83\%A, H51-F1=0.697\%A, H53-F1=1.179\%A. this is nonsensical

DIFMX_01  Alert B The maximum difference density is > 0.1*ZMAX*1.00
          _refine_diff_density_max given =      0.830
          Test value =      0.700

Author response: ... The corresponding peak F2 has been loacated near H(51) : C5-F2=0.688\%A, H51-F2=0.567\%A and H51-F2=1.264\%A. this is nonsensical

 Alert Level C:



CELLK_01  Alert C Check that the cell measurement temperature is in Kelvin.
          Value of measurement temperature given =    20.000

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.

REFNR_01  Alert C Ratio of reflections to parameters is < 10 for a
          centrosymmetric structure
          sine(theta)/lambda                0.7327
          Proportion of unique data used    0.6971
          Ratio reflections to parameters   9.1575

General Notes



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


 0 Alert Level A = Potentially serious problem

 2 Alert Level B = Potential problem

 4 Alert Level C = Please check

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S1. Comment

Light particles experiencing potential functions with topological degeneracy manifest their quantum nature via tunnelling. The magnitude of the tunnel splitting depends on the particle mass and potential shape (distances between identical sites and barrier height). For methyl groups the threefold symmetry ensures strict topological degeneracy and rotational tunnelling has been observed with inelastic neutron-scattering techniques (INS) in many crystals (Press, 1981; Prager & Heidemann, 1995). The INS spectrum of DMP at 2 K exhibits tunnelling lines at 20 and 29 µeV (Nicolaï, Kaiser et al., 1998). The presence of more than one tunnelling transition in the same system is rather rare. It reveals either inequivalent methyl groups or dynamical coupling. A precise knowledge of the crystalline structure at the same temperature of the tunnelling measurements is necessary to interpret the INS spectra (Johnson et al., 1996; Johnson et al., 1997; Neumann & Johnson, 1997; Nicolaï, Kearley et al., 1998). We present in this paper the structure determination at 20 K. The structure at the temperature of the tunnelling experiment, 5 K, is presented in the following article (Kaiser-Morris et al., 2001).

The space group P2₁/a (monoclinic) with four formula units per unit cell was obtained from a preliminary study at 253 K with X-rays (Morris et al., 1998) and a recent X-ray diffraction study at 180 K with a better resolution (Thalladi et al., 2000). A preliminary neutron-diffraction experiment at 260 K, close to the melting point of the title compound, and neutron diffraction experiments on a single-crystal at 20 and 5 K confirmed the space group P2₁/a (Fig. 1). There is no evidence for any phase transition between 260 and 20 K, but there are significant changes of the lattice parameters below 260 K: at 260 K, the cell parameters are: a = 7.467 (8), b = 10.859 (7), c = 7.558 (7) Å, β = 90.8 (4)°.

S2. Experimental

2,6-Dimethylpyrazine (DMP) is hygroscopic and melts at 311 K. We performed neutron diffraction experiments with a single-crystal at 260, 20 and 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 wavelength 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 crystal structure of (I) at 20 K. For a single molecule representation, see the following 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 = 7.288 (5) Å	Cell parameters from 16 reflections
b = 10.73 (1) Å	θ = 9.8–21.5°
c = 7.444 (7) Å	µ = 0.08 mm^−1
β = 90.10 (8)°	T = 20 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 = 0.074
Radiation source: Orphée reactor Saclay France	θmax = 37.5°, θmin = 1°
Cu (220) monochromator	h = −10→2
ω scans	k = 0→15
2230 measured reflections	l = −10→10
1918 independent reflections	2 standard reflections every 450 min
1337 reflections with I > 3σ(I)	intensity decay: none

Refinement

Refinement on F	All H-atom parameters refined
Least-squares matrix: full	Chebychev polynomial with 5 parameters: 1.04, -3.06, -0.108, -0.644, -0.800 (Carruthers & Watkin, 1979)
R[F^2 > 2σ(F^2)] = 0.031	(Δ/σ)max = 0.007
wR(F^2) = 0.019	Δρmax = 0.83 e Å^−3
S = 1.03	Δρmin = −0.89 e Å^−3
1337 reflections	Extinction correction: Larson, 1970
146 parameters	Extinction coefficient: 1.52 (13)

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

	x	y	z	Uiso*/Ueq
N1	0.12257 (8)	0.98276 (5)	0.23638 (8)	0.0070
N2	0.19670 (7)	0.74325 (5)	0.36768 (7)	0.0057
C1	0.19500 (11)	0.96465 (7)	0.3992 (1)	0.0062
C2	0.23295 (11)	0.84500 (7)	0.4658 (1)	0.0052
C3	0.1245 (1)	0.76032 (7)	0.20383 (11)	0.0049
C4	0.08847 (11)	0.88000 (7)	0.13941 (11)	0.0060
C5	0.31567 (12)	0.82858 (8)	0.64895 (11)	0.0080
C6	0.08306 (11)	0.64661 (8)	0.09342 (11)	0.0079
H11	0.2239 (3)	1.04718 (17)	0.4806 (3)	0.0215
H41	0.0295 (3)	0.8933 (2)	0.0052 (3)	0.0205
H51	0.3174 (6)	0.7323 (2)	0.6885 (4)	0.0437
H52	0.4564 (4)	0.8622 (4)	0.6503 (4)	0.0447
H53	0.2403 (5)	0.8820 (3)	0.7480 (3)	0.0439
H61	0.0101 (6)	0.6703 (3)	−0.0285 (4)	0.0422
H62	0.2068 (3)	0.5988 (3)	0.0548 (5)	0.0441
H63	0.0000 (5)	0.5815 (2)	0.1698 (4)	0.0374

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0092 (2)	0.0050 (2)	0.0067 (2)	0.00117 (18)	−0.00155 (17)	0.00074 (17)
N2	0.0075 (2)	0.0043 (2)	0.0052 (2)	0.00022 (17)	−0.00173 (19)	0.00042 (16)
C1	0.0090 (3)	0.0033 (3)	0.0062 (3)	0.0001 (2)	−0.0013 (2)	−0.0004 (2)
C2	0.0071 (3)	0.0046 (3)	0.0041 (3)	−0.0003 (2)	−0.0014 (2)	0.0002 (2)
C3	0.0058 (3)	0.0048 (3)	0.0042 (3)	−0.0004 (2)	−0.0013 (2)	−0.0001 (2)
C4	0.0073 (3)	0.0052 (3)	0.0053 (3)	0.0002 (2)	−0.0017 (2)	0.0010 (2)
C5	0.0111 (3)	0.0075 (3)	0.0054 (3)	−0.0002 (3)	−0.0035 (3)	−0.0002 (2)
C6	0.0087 (3)	0.0067 (3)	0.0083 (3)	−0.0007 (3)	−0.0021 (3)	−0.0020 (3)
H11	0.032 (1)	0.0122 (7)	0.0198 (8)	0.0016 (7)	−0.0060 (7)	−0.0047 (6)
H41	0.0271 (9)	0.0196 (8)	0.0147 (7)	0.0013 (7)	−0.0088 (7)	0.0026 (6)
H51	0.081 (2)	0.0180 (9)	0.0319 (13)	−0.0042 (12)	−0.0274 (14)	0.0070 (8)
H52	0.025 (1)	0.078 (2)	0.0317 (11)	−0.0182 (13)	−0.0105 (9)	0.0152 (13)
H53	0.0547 (16)	0.0587 (18)	0.0184 (9)	0.0308 (15)	−0.001 (1)	−0.011 (1)
H61	0.070 (2)	0.0286 (12)	0.0280 (11)	0.0020 (12)	−0.0292 (13)	−0.0041 (9)
H62	0.021 (1)	0.0407 (14)	0.071 (2)	0.0046 (9)	0.0017 (11)	−0.0339 (14)
H63	0.0537 (16)	0.0260 (11)	0.0325 (11)	−0.0220 (11)	0.0127 (11)	−0.0061 (8)

Geometric parameters (Å, º)

N1—C1	1.3354 (9)	C3—C6	1.5019 (11)
N1—C4	1.341 (1)	C4—H41	1.0963 (19)
N2—C2	1.3399 (9)	C5—H51	1.074 (2)
N2—C3	1.340 (1)	C5—H52	1.087 (3)
C1—C2	1.4040 (11)	C5—H53	1.084 (3)
C1—H11	1.0937 (19)	C6—H61	1.081 (3)
C2—C5	1.5005 (11)	C6—H62	1.078 (2)
C3—C4	1.3960 (11)	C6—H63	1.086 (3)
C1—N1—C4	116.19 (6)	C3—C4—H41	120.43 (13)
C2—N2—C3	117.47 (6)	C2—C5—H51	111.49 (16)
N1—C1—C2	122.06 (7)	C2—C5—H52	110.32 (15)
N1—C1—H11	117.42 (13)	H51—C5—H52	107.8 (3)
C2—C1—H11	120.51 (13)	C2—C5—H53	110.67 (16)
N2—C2—C1	120.99 (7)	H51—C5—H53	109.1 (3)
N2—C2—C5	118.56 (7)	H52—C5—H53	107.3 (3)
C1—C2—C5	120.45 (7)	C3—C6—H61	111.48 (16)
N2—C3—C4	120.78 (7)	C3—C6—H62	111.47 (15)
N2—C3—C6	117.72 (7)	H61—C6—H62	107.4 (3)
C4—C3—C6	121.50 (7)	C3—C6—H63	110.40 (15)
N1—C4—C3	122.51 (7)	H61—C6—H63	108.5 (3)
N1—C4—H41	117.06 (13)	H62—C6—H63	107.5 (3)