2,6-Di-tert-butyl­phenol revisited at 110 K

The title compound, C₁₄H₂₂O, was studied at 110 K. The phenolic hydroxy group was found to be coplanar with the benzene ring and, due to steric hindrance from the tert-butyl groups, this hydroxy group does not form hydrogen bonds. The shortest inter­molecular O...O distance is 3.1008 (11) Å, with an O—H...O angle of 117.3 (16)°. There are no significant inter­molecular π–π stacking or C—H...π inter­actions.

S1. Comment

Our interest in the structure of the title compound (I) was aroused by the crystal structure of 5-(3,5-di-tert-butyl-4-hydroxybenzylidene)-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione [compound (II); compound (V) in Rezende et al., 2005]. Compound (II) features an OH moiety that does not appear to form a hydrogen bond, with closest O···O distances of 3.391 (2) and 3.010 (2) Å for the two independent molecules in (II). Cases like this are rare. The Cambridge Structural Database (CSD, Version?; Allen, 2002) was analysed for similar configurations. In most cases where OH groups do not donate hydrogen bonds, this is due to missing or misplaced H atoms. Only in sterically very congested situations is hydrogen bonding really impossible (see below).

Substituted phenols have a technical application as antioxidants. In the first step of the oxidation reaction, the phenolic H atom is released and a phenoxyl radical is formed. It is well known that the strength of the O—H bond is weaker if bulky substituents like tert-butyl groups are introduced in ortho positions, and also that the electronic influence of para-substituents is important (Lucarini et al., 1996). Therefore, it is essential to know the conformation of the OH group in (I), which has two bulky substituents in the ortho positions and only an H atom in the para position.

The title compound, (I), is related to (II) and its structure has been published previously by Lazarev et al. (1992) based on a room-temperature data collection. The H-atom position of the phenolic hydroxy group could not be determined in that structure. In order to determine this H-atom position reliably, we redetermined the structure at 110 K. All H atoms could be located in the difference Fourier map and were refined freely with isotropic displacement parameters.

The hydroxy group is in the plane of the phenyl ring, with a C6—C1—O1—H1O torsion angle of −0.4 (14)° (Fig. 1). This finding is in accordance with high-level ab initio calculations (Ribeiro da Silva et al., 1999) and with the crystal structures of most non-substituted phenols. The planarity is also consistent with the findings in the monoclinic form of 2,6-di-tert-butyl-4-methylphenol (space group C2/c, CSD refcodes MBPHOL01 and MBPHOL11), where the OH torsion angle is −1 or 4°, respectively, but the CSD contains additional polymorphs with OH torsion angles up to 30 and 26°, respectively (orthorhombic form, space group P2₁2₁2₁, CSD refcodes MBPHOL02 and MBPHOL10). A second monoclinic polymorph (space group Cc, CSD refcode MBPHOL12) has a torsion angle of −3°. Contrary to our present results for (I), polarizability studies (Aroney et al., 1964), ESR experiments (Nemoto et al., 1981) and semi-empirical AM1 calculations (Brewster et al., 1994) suggest a nonplanarity of the OH group in (I).

Molecules of (I) have an approximate non-crystallographic mirror symmetry, with the phenyl ring and the OH group in the mirror plane with an r.m.s. deviation (Pilati & Forni, 1998) of 0.0328 Å. When the H atoms are ignored from this calculation, the molecule has an approximate C_2v symmetry with an r.m.s. deviation of 0.0273 Å. A rigid-body analysis can be performed on the molecule using the program THMA11 (Schomaker & Trueblood, 1998). When the two tert-butyl groups are defined as independent rotors, low rigid-body R values of 0.062 (for all U values) and 0.046 (for diagonal U values) can be achieved. The C1—O1 distance corrected for rigid-body motion is 1.390 Å, compared with the uncorrected value of 1.3865 (12) Å.

The coplanarity of the OH group with the phenyl ring leads to very short intramolecular H···H contacts with one of the tert-butyl groups. The shortest H···H distances are 1.90 (2) Å to H14C and 1.92 (2) Å to H12C. Accordingly, the C11—C14 and C11—C12 distances of 1.5466 (15) and 1.5480 (16) Å, respectively, are the longest C—C bonds of the tert-butyl groups. The congestion around the OH group can also be seen in the C—C—O bond angles of 120.59 (9)° in the direction of the H···H interactions and 116.45 (8)° in the opposite direction, showing that the O atom is slightly bent away. These H···H interactions are certainly a reason for the weakness of the O—H bond.

Due to the steric shielding of the two tert-butyl groups, there is no intermolecular hydrogen bonding involving the OH group. Thus, the shortest intermolecular O···O distance is 3.1008 (11) Å, with an O—H···O angle of 117.3 (16)° (Fig. 2). As there are no hydrogen-bond acceptors in the molecule (apart from the phenolic OH itself), no intramolecular hydrogen bonding is present. Significant intermolecular π—π stacking and C—H···π interactions are not observed. Consequently, the density of the crystal is rather low and the packing index (Kitajgorodskij, 1973) is only 67.5%, which is on the lower end of the 65–75% range expected for organic solids (Dunitz, 1995).

A study of the CSD (update August 2005) gives 90 entries with one or more 2,6-di-tert-butyl-substituted phenol moieties. In 34 of these entries, the coordinates of the OH H atoms are not deposited on the CSD, and three additional entries were not considered because of disorder. Of the remaining 53 entries, 29 do not form any hydrogen bonds with the shielded OH group as donor. In 24 entries, the shielded OH group is a hydrogen-bond donor with various acceptor atoms in the crystal structure. In none of the 90 entries is there a hydrogen bond between shielded OH groups.

S2. Experimental

Single crystals for the diffraction experiment were obtained by slow evaporation of a solution of 2,6-di-tert-butylphenol (ACROS Organics) in ethanol.

S3. Refinement

The X-ray intensities were obtained with two different exposure times and rotation angles of 1°. 364 ϕ and 414 ω scans were measured with an exposure time of 40 s per frame, and 156 ϕ scans with an exposure time of 8 s per frame. All C-bound H atoms were refined freely with isotropic displacement parameters in order to demonstrate that the outcome of the free refinement of H1O is reliable.

Computing details

Data collection: COLLECT (Nonius, 1999); cell refinement: PEAKREF (Schreurs, 2005); data reduction: EVALCCD (Duisenberg et al. 2003); program(s) used to solve structure: coordinates adapted from literature (Lazarev et al., 1992); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: PLATON.

Figures

	Fig. 1. Displacement ellipsoid plot of (I). Ellipsoids are drawn at the 50% probability level and H atoms are drawn as small spheres of arbitrary radii.
	Fig. 2. The packing arrangement of (I) in the crystal, which is unfavourable for hydrogen bonding. [Symmetry code: (i) 1 − x, −y, 1 − z.]

2,6-Di-tert-butylphenol

Crystal data

C14H22O	F(000) = 456
Mr = 206.32	Dx = 1.080 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 13875 reflections
a = 7.1369 (2) Å	θ = 2.2–27.5°
b = 18.6755 (6) Å	µ = 0.07 mm^−1
c = 9.8009 (4) Å	T = 110 K
β = 103.694 (3)°	Plate, yellow
V = 1269.19 (8) Å^3	0.48 × 0.48 × 0.06 mm
Z = 4

Data collection

Nonius KappaCCD area-detector diffractometer	2912 independent reflections
Radiation source: rotating anode	2409 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.031
ϕ and ω scans	θmax = 27.5°, θmin = 2.2°
Absorption correction: multi-scan (SADABS; Sheldrick, 2002)	h = −9→9
Tmin = 0.78, Tmax = 1.0	k = −23→24
27689 measured reflections	l = −12→12

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.037	Hydrogen site location: difference Fourier map
wR(F^2) = 0.100	All H-atom parameters refined
S = 1.03	w = 1/[σ^2(Fo^2) + (0.0471P)^2 + 0.4344P] where P = (Fo^2 + 2Fc^2)/3
2912 reflections	(Δ/σ)max = 0.001
224 parameters	Δρmax = 0.33 e Å^−3
0 restraints	Δρmin = −0.18 e Å^−3

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. The weighted R-factor wR and goodness 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
O1	0.59379 (11)	0.06999 (4)	0.46293 (8)	0.02165 (19)
C1	0.75172 (14)	0.11423 (5)	0.46966 (10)	0.0158 (2)
C2	0.75369 (14)	0.15427 (5)	0.34740 (10)	0.0159 (2)
C3	0.91446 (16)	0.19748 (6)	0.35162 (11)	0.0199 (2)
C4	1.06521 (16)	0.20153 (6)	0.47096 (12)	0.0219 (2)
C5	1.05789 (15)	0.16235 (6)	0.58973 (11)	0.0201 (2)
C6	0.90145 (14)	0.11771 (5)	0.59324 (10)	0.0166 (2)
C7	0.58678 (15)	0.15051 (6)	0.21439 (10)	0.0184 (2)
C8	0.39702 (16)	0.17560 (7)	0.24882 (12)	0.0245 (2)
C9	0.62340 (19)	0.19935 (6)	0.09685 (12)	0.0256 (3)
C10	0.56499 (17)	0.07353 (6)	0.15534 (11)	0.0224 (2)
C11	0.89510 (16)	0.07443 (6)	0.72660 (11)	0.0197 (2)
C12	0.71931 (19)	0.09670 (7)	0.78393 (12)	0.0266 (3)
C13	1.07468 (19)	0.08910 (7)	0.84618 (12)	0.0289 (3)
C14	0.89455 (18)	−0.00696 (6)	0.69739 (12)	0.0241 (2)
H1O	0.606 (3)	0.0479 (10)	0.540 (2)	0.056 (5)*
H3	0.9234 (19)	0.2256 (7)	0.2717 (14)	0.024 (3)*
H4	1.1758 (19)	0.2320 (7)	0.4714 (14)	0.024 (3)*
H5	1.166 (2)	0.1657 (7)	0.6725 (14)	0.027 (3)*
H8A	0.362 (2)	0.1468 (7)	0.3251 (15)	0.028 (3)*
H8B	0.289 (2)	0.1731 (8)	0.1619 (16)	0.035 (4)*
H8C	0.407 (2)	0.2261 (8)	0.2783 (15)	0.030 (4)*
H9A	0.630 (2)	0.2506 (9)	0.1242 (16)	0.037 (4)*
H9B	0.513 (2)	0.1930 (7)	0.0130 (15)	0.031 (4)*
H9C	0.743 (2)	0.1856 (7)	0.0680 (14)	0.027 (3)*
H10A	0.458 (2)	0.0717 (7)	0.0700 (16)	0.033 (4)*
H10B	0.683 (2)	0.0590 (7)	0.1280 (15)	0.030 (3)*
H10C	0.540 (2)	0.0379 (8)	0.2267 (15)	0.031 (4)*
H12A	0.729 (2)	0.1485 (8)	0.8103 (15)	0.037 (4)*
H12B	0.718 (2)	0.0677 (8)	0.8703 (16)	0.036 (4)*
H12C	0.589 (2)	0.0893 (8)	0.7161 (16)	0.037 (4)*
H13A	1.083 (2)	0.1410 (8)	0.8764 (15)	0.035 (4)*
H13B	1.064 (2)	0.0587 (8)	0.9300 (16)	0.036 (4)*
H13C	1.200 (2)	0.0739 (8)	0.8194 (16)	0.037 (4)*
H14A	0.902 (2)	−0.0345 (8)	0.7852 (16)	0.034 (4)*
H14B	1.008 (2)	−0.0198 (8)	0.6609 (15)	0.035 (4)*
H14C	0.780 (2)	−0.0250 (8)	0.6269 (16)	0.036 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0206 (4)	0.0265 (4)	0.0166 (4)	−0.0068 (3)	0.0018 (3)	0.0055 (3)
C1	0.0164 (5)	0.0152 (5)	0.0163 (5)	−0.0006 (4)	0.0051 (4)	−0.0015 (4)
C2	0.0195 (5)	0.0155 (5)	0.0133 (5)	0.0013 (4)	0.0051 (4)	−0.0015 (4)
C3	0.0255 (5)	0.0187 (5)	0.0178 (5)	−0.0025 (4)	0.0096 (4)	−0.0001 (4)
C4	0.0213 (5)	0.0213 (5)	0.0244 (5)	−0.0055 (4)	0.0082 (4)	−0.0041 (4)
C5	0.0194 (5)	0.0203 (5)	0.0194 (5)	−0.0012 (4)	0.0021 (4)	−0.0042 (4)
C6	0.0201 (5)	0.0152 (5)	0.0146 (5)	0.0018 (4)	0.0042 (4)	−0.0016 (4)
C7	0.0221 (5)	0.0186 (5)	0.0140 (5)	0.0006 (4)	0.0033 (4)	0.0013 (4)
C8	0.0232 (5)	0.0277 (6)	0.0217 (5)	0.0056 (4)	0.0034 (4)	0.0024 (5)
C9	0.0354 (6)	0.0253 (6)	0.0154 (5)	−0.0015 (5)	0.0043 (5)	0.0042 (4)
C10	0.0267 (6)	0.0215 (5)	0.0167 (5)	−0.0003 (4)	0.0008 (4)	−0.0013 (4)
C11	0.0260 (5)	0.0180 (5)	0.0134 (5)	−0.0001 (4)	0.0015 (4)	0.0006 (4)
C12	0.0389 (7)	0.0258 (6)	0.0182 (5)	0.0008 (5)	0.0127 (5)	0.0009 (4)
C13	0.0376 (7)	0.0248 (6)	0.0183 (5)	−0.0036 (5)	−0.0057 (5)	0.0014 (4)
C14	0.0302 (6)	0.0182 (5)	0.0209 (5)	0.0010 (4)	0.0000 (5)	0.0004 (4)

Geometric parameters (Å, º)

O1—C1	1.3865 (12)	C9—H9A	0.992 (16)
O1—H1O	0.845 (19)	C9—H9B	1.003 (14)
C1—C6	1.4144 (13)	C9—H9C	0.996 (14)
C1—C2	1.4152 (14)	C10—H10A	0.990 (15)
C2—C3	1.3954 (14)	C10—H10B	0.982 (15)
C2—C7	1.5453 (14)	C10—H10C	1.010 (15)
C3—C4	1.3913 (15)	C11—C13	1.5427 (15)
C3—H3	0.958 (14)	C11—C14	1.5466 (15)
C4—C5	1.3863 (15)	C11—C12	1.5480 (16)
C4—H4	0.972 (14)	C12—H12A	1.000 (15)
C5—C6	1.4003 (14)	C12—H12B	1.007 (16)
C5—H5	0.982 (14)	C12—H12C	1.017 (15)
C6—C11	1.5466 (14)	C13—H13A	1.012 (15)
C7—C9	1.5394 (15)	C13—H13B	1.015 (15)
C7—C10	1.5439 (15)	C13—H13C	1.029 (16)
C7—C8	1.5441 (15)	C14—H14A	0.993 (15)
C8—H8A	0.999 (14)	C14—H14B	0.989 (16)
C8—H8B	1.006 (15)	C14—H14C	0.995 (15)
C8—H8C	0.985 (15)
C1—O1—H1O	109.7 (12)	C7—C9—H9C	111.7 (8)
O1—C1—C6	120.59 (9)	H9A—C9—H9C	109.7 (12)
O1—C1—C2	116.45 (8)	H9B—C9—H9C	107.4 (11)
C6—C1—C2	122.95 (9)	C7—C10—H10A	109.8 (8)
C3—C2—C1	116.96 (9)	C7—C10—H10B	110.0 (8)
C3—C2—C7	121.03 (9)	H10A—C10—H10B	107.4 (11)
C1—C2—C7	122.02 (9)	C7—C10—H10C	111.9 (8)
C4—C3—C2	121.55 (10)	H10A—C10—H10C	109.7 (11)
C4—C3—H3	118.0 (8)	H10B—C10—H10C	107.9 (11)
C2—C3—H3	120.4 (8)	C13—C11—C14	106.28 (9)
C5—C4—C3	120.12 (10)	C13—C11—C6	111.34 (9)
C5—C4—H4	119.8 (8)	C14—C11—C6	110.88 (9)
C3—C4—H4	120.1 (8)	C13—C11—C12	105.99 (9)
C4—C5—C6	121.58 (10)	C14—C11—C12	111.38 (9)
C4—C5—H5	118.7 (8)	C6—C11—C12	110.78 (8)
C6—C5—H5	119.7 (8)	C11—C12—H12A	109.9 (9)
C5—C6—C1	116.84 (9)	C11—C12—H12B	109.3 (8)
C5—C6—C11	121.00 (9)	H12A—C12—H12B	108.5 (12)
C1—C6—C11	122.17 (9)	C11—C12—H12C	114.9 (9)
C9—C7—C10	106.93 (9)	H12A—C12—H12C	107.3 (12)
C9—C7—C8	107.27 (9)	H12B—C12—H12C	106.6 (12)
C10—C7—C8	110.41 (9)	C11—C13—H13A	112.0 (8)
C9—C7—C2	111.77 (9)	C11—C13—H13B	108.0 (8)
C10—C7—C2	110.34 (8)	H13A—C13—H13B	108.0 (12)
C8—C7—C2	110.04 (8)	C11—C13—H13C	111.9 (8)
C7—C8—H8A	112.7 (8)	H13A—C13—H13C	110.0 (12)
C7—C8—H8B	109.7 (8)	H13B—C13—H13C	106.7 (12)
H8A—C8—H8B	109.4 (11)	C11—C14—H14A	110.6 (9)
C7—C8—H8C	110.3 (8)	C11—C14—H14B	109.8 (9)
H8A—C8—H8C	108.1 (11)	H14A—C14—H14B	107.7 (12)
H8B—C8—H8C	106.4 (11)	C11—C14—H14C	115.3 (8)
C7—C9—H9A	112.0 (9)	H14A—C14—H14C	107.1 (12)
C7—C9—H9B	107.6 (8)	H14B—C14—H14C	106.0 (12)
H9A—C9—H9B	108.2 (12)
O1—C1—C2—C3	−178.28 (9)	C2—C1—C6—C11	179.01 (9)
C6—C1—C2—C3	1.28 (15)	C3—C2—C7—C9	−0.64 (13)
O1—C1—C2—C7	1.45 (14)	C1—C2—C7—C9	179.64 (9)
C6—C1—C2—C7	−178.99 (9)	C3—C2—C7—C10	118.21 (11)
C1—C2—C3—C4	−0.54 (15)	C1—C2—C7—C10	−61.51 (12)
C7—C2—C3—C4	179.73 (10)	C3—C2—C7—C8	−119.71 (11)
C2—C3—C4—C5	−0.27 (16)	C1—C2—C7—C8	60.57 (12)
C3—C4—C5—C6	0.40 (16)	C5—C6—C11—C13	1.08 (14)
C4—C5—C6—C1	0.30 (15)	C1—C6—C11—C13	−179.10 (9)
C4—C5—C6—C11	−179.88 (10)	C5—C6—C11—C14	−117.04 (11)
O1—C1—C6—C5	178.38 (9)	C1—C6—C11—C14	62.78 (13)
C2—C1—C6—C5	−1.16 (14)	C5—C6—C11—C12	118.77 (11)
O1—C1—C6—C11	−1.44 (14)	C1—C6—C11—C12	−61.42 (12)