2-(4-Hydroxy­phenyl)-4,4-dimethyl-2-oxazoline: X-ray and density functional theory study

In the crystal structure of the title compound, C₁₁H₁₃NO₂, there are strong inter­molecular O—H...N hydrogen bonds which, together with weak intra­molecular C—H...O hydrogen bonds, lead to the formation of infinite chains of mol­ecules, held together by weak inter­molecular C—H...O hydrogen bonds. A theoretical investigation of the hydrogen bonding, based on density functional theory (DFT) employing periodic boundary conditions, is in agreement with the experimental data. The cluster approach shows that the influence of the crystal field and of hydrogen-bond formation are responsible for the deformation of the 2-oxazoline ring, which is not planar and adopts a ⁴T₃ (^C3T_C2) conformation.

S1. Comment

In a previous paper (Langer et al., 2005), we reported crystal structures for three isomers of (hydroxyphenyl)-2-oxazoline, namely 2-(2-hydroxyphenyl)-2-oxazoline, 2-(3-hydroxyphenyl)-2-oxazoline and 2-(4-hydroxyphenyl)-2-oxazoline. All these compounds polymerize rapidly at around 473 K. On the other hand, it has been observed that polymerization of 2-(4-hydroxyphenyl)-4,4-dimethyl-2-oxazoline, (I), is restricted under similar conditions (Lustoň et al., 2006). This confirmed the observation of Bassiri et al. (1967) that 4- and 5-substituted 2-oxazolines have low reactivity in the course of cationic polymerization. Therefore, it was of interest to determine the crystal structure of (I) as a contribution to the understanding of low reactivity in both ring-opening polyaddition reactions and in cationic polymerization.

The numbering scheme, together with the corresponding atomic displacement ellipsoid plot for (I), are shown in Fig. 1. Selected geometric parameters for (I) are listed in Table 1. The C1—C4 bond in (I) is significantly shorter than the usual value for a single C—C bond (Reference for standard value?), indicating weak conjugation between the 2-oxazoline ring substituted at the C-2 position (C1) and the benzene ring. The angle between the planes of the oxazoline and benzene rings is 5.84 (8)°. Surprisingly, the 2-oxazoline ring in (I) is not planar, as it is for example for 2-(2-hydroxyphenyl)-2-oxazoline and 2-(4-hydroxyphenyl)-2-oxazoline (Langer et al., 2005). The values of the relevant dihedral angles (Table 1) and puckering parameters (Cremer & Pople, 1975), Q = 0.142 (2) Å and ϕ = 134.4 (6)°, indicate that this ring deviates significantly from planarity (twisting on C2—C3) and adopts a ⁴T₃ (^C3T_C2) conformation slightly distorted towards the ⁴E (^C3E) conformation. This is similar to what was found for one of the symmetry-independent molecules in the structure of 2-(3-hydroxyphenyl)-2-oxazoline (Langer et al., 2005), but in that case a ³T₄ conformation of the 2-oxazoline ring was observed. We assume that the presence of a 4,4-dimethyl substituent is responsible for the observed deviation from planarity and hence the observed low reactivity in the course of both thermally initiated polyaddition and cationic polymerization of the title compound.

Density functional theory (DFT) calculations at the B3LYP/6–31G** level of theory using GAUSSIAN98 (Frisch et al., 1998) were used in an attempt to explain the non-planarity of the 2-oxazoline ring in (I). An isolated molecule of (I) was optimized first and, surprisingly, the molecule with a planar 2-oxazoline ring was the stable configuration. In order to simulate the influence of nearest neighbours on the molecular geometry, a methanol molecule was positioned to allow formation of the O2—H2···N1 hydrogen bond. In this case, the molecular geometry remains very close to that found in the crystal structure. The calculated O2—H2···N1 hydrogen bond (1.89 Å) and D—H···A angle (173°) agree well with the experimental data (1.88 Å, 174°). A trial with the molecule having a planar oxazoline ring and a methanol molecule in the neighbourhood also resulted in the deformation of the oxazoline ring. The calculated O2—H2···N1 hydrogen-bond energy of a molecule of (I) with a methanol molecule, corrected for the basis set superposition error (BSSE) using a standard procedure (Boys & Bernardi, 1970), was estimated to be approximately −13 kJ mol⁻¹. This value falls within the range of weak hydrogen bonds (Jeffrey, 1997). Despite the simplicity of such simulations, we propose that it is the influence of the crystal field and hydrogen-bond formation which are responsible for the deformation of the oxazoline ring. Llamas-Saiz et al. (1992) published a statistical survey of the R—O—H···Nsp² intermolecular interaction in organic crystals. The calculated geometric parameters presented in this work (Table 2) agree with their published mean values (in parentheses?) [O—H bond 1.06 Å (0.97 Å), O···N 2.62 Å (2.80 Å), H···N 1.56 Å (1.87 Å) and O—H···N hydrogen-bond angle 173° (175°)].

The hydrogen-bonding geometry for (I) is listed in Table 2. A theoretical investigation of hydrogen bonds was performed using the Vienna ab initio simulation package VASP (Kresse & Furthmüller, 1996; Kresse & Hafner, 1993). The calculations were based on DFT with periodic boundary conditions (Jones & Gunnarsson, 1989) using a generalized gradient approximation (GGA) in an exchange-correlation function (Perdew et al., 1992). The interactions between ions and electrons were described using the projector-augemented wave method (PAW; Kresse & Joubert, 1999) with a plane wave cut-off of 400 eV. The optimizations of the structure were carried out using the conjugated gradient method in 4κ points (Teter et al., 1989; Bylander et al., 1990). The results of the calculations are in agreement with the experimental data.

The hydrogen-bonding pattern can be described using graph theory (Bernstein et al., 1995; Grell et al., 1999). For (I), there is just one intermolecular hydrogen bond, (a), of the O—H···N type, which is described as a C(8) chain in the first-level graph set (Fig. 2). There are also two weak hydrogen bonds of C—H···O type, one intramolecular, (b), described as an S(5) string, and one intermolecular, (c), described as a C(5) chain. On the second-level graph-set theory, C₂²(7) and C₂²(13) chains, formed by hydrogen bonds (a) and (c), are recognized. The assignment of graph-set descriptors was performed using PLUTO, as described by Motherwell et al. (1999) and the notation of the hydrogen bonds here is that given in Table 2.

S2. Experimental

Compound (I) was prepared as follows (see scheme). Methyl 4-hydroxybenzoate (7.61 g, 0.05 mol) and 2-amino-2-methyl-1-propanol (4.46 g, 0.05 mol) were stirred under argon at 423 K for 6 h and evolved methanol was collected in a Dean–Stark trap. The product, N-(1,1-dimethyl-2-hydroxyethyl)-4-hydroxybenzamide, a brownish viscous material, was diluted with CH₂Cl₂ (50 ml) and cyclized in the next step with thionyl chloride (11.2 g, 0.1 mol, added dropwise at 273 K). The reaction mixture was then left for 48 h at room temperature. A brownish solid was isolated, washed with CH₂Cl₂ and dried. It was then dispersed in water and solid NaHCO₃ was added in small portions until the pH was neutral. The resulting white solid, which appeared in several minutes, was recrystallized from toluene. Colourless crystals of compound (I) (m.p. 494–496 K) were obtained (yield 8.02 g, 83.9%). Spectroscopic analysis: ¹H NMR (DMSO-d₆, δ, p.p.m.): 10.04 (s, 1H, OH), 7.66 (d, 2H, ar.), 6.80 (d, 2H, ar.), 4.03 (s, 2H, OCH₂), 1.24 (s, 6H, CH₃).

S3. Refinement

H atoms were allowed for isotropically and were constrained to ideal geometry using an appropriate riding model. The C—H distance was kept fixed at 0.95 Å for aromatic H atoms and at 0.99 Å for secondary H atoms. For the hydroxyl group, the O—H distance (0.84 Å) and C—O—H angle (109.5°) were kept fixed, while the torsion angle was allowed to refine, with the starting position based on the circular Fourier synthesis. For methyl groups, the C—H distances (0.98 Å) and C—C—H angles (109.5°) were kept fixed, while the torsion angles were allowed to refine with the starting position based on the threefold averaged circular Fourier synthesis.

Computing details

Data collection: SMART (Siemens, 1995); cell refinement: SAINT (Siemens, 1995); data reduction: SAINT and SADABS (Sheldrick, 2002); program(s) used to solve structure: SHELXTL (Bruker, 2001); program(s) used to refine structure: SHELXTL; molecular graphics: DIAMOND (Brandenburg, 2005); software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. The numbering scheme of (I), with atomic displacement ellipsoids drawn at the 50% probability level.
	Fig. 2. The hydrogen-bonding pattern in the crystal structure of (I), viewed along the b axis. H atoms not involved in hydrogen bonding and methyl C atoms have been omitted for clarity. For symbols and symmetry codes, see Table 2.

2-(4-Hydroxyphenyl)-4,4-dimethyl-2-oxazoline

Crystal data

C11H13NO2	F(000) = 408
Mr = 191.22	Dx = 1.219 Mg m^−3
Orthorhombic, Pca21	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2c -2ac	Cell parameters from 7449 reflections
a = 12.1590 (2) Å	θ = 2.9–32.9°
b = 8.5666 (1) Å	µ = 0.08 mm^−1
c = 9.9999 (1) Å	T = 173 K
V = 1041.60 (2) Å^3	Block, colourless
Z = 4	0.80 × 0.40 × 0.26 mm

Data collection

Siemens SMART CCD area-detector diffractometer	1962 independent reflections
Radiation source: fine-focus sealed tube	1776 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.029
ω scans	θmax = 32.9°, θmin = 2.9°
Absorption correction: multi-scan (SADABS; Sheldrick, 2002)	h = −18→17
Tmin = 0.936, Tmax = 0.978	k = −12→11
11104 measured reflections	l = −14→15

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)] = 0.036	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.105	H-atom parameters constrained
S = 1.05	w = 1/[σ^2(Fo^2) + (0.0709P)^2 + 0.0539P] where P = (Fo^2 + 2Fc^2)/3
1962 reflections	(Δ/σ)max = 0.001
138 parameters	Δρmax = 0.28 e Å^−3
1 restraint	Δρmin = −0.18 e Å^−3

Special details

Experimental. Data were collected at low temperature using a Siemens SMART CCD diffractometer equiped with a LT-2 device. A full sphere of reciprocal space was scanned by 0.3° steps in ω with a crystal–to–detector distance of 3.97 cm, 15 s per frame. Preliminary orientation matrix was obtained from the first 100 frames using SMART (Siemens, 1995). The collected frames were integrated using the preliminary orientation matrix which was updated every 100 frames. Final cell parameters were obtained by refinement on the position of 7449 reflections with I>10σ(I) after integration of all the frames data using SAINT (Siemens, 1995).

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	0.23144 (8)	0.16534 (12)	0.46604 (12)	0.0302 (2)
O2	0.02632 (8)	0.72996 (13)	0.14001 (11)	0.0295 (2)
H2	−0.0265	0.7533	0.1904	0.071 (9)*
N1	0.36243 (9)	0.17600 (13)	0.30627 (13)	0.0257 (2)
C1	0.27242 (9)	0.23219 (15)	0.35271 (14)	0.0224 (2)
C2	0.39451 (12)	0.04170 (17)	0.39290 (16)	0.0289 (3)
C3	0.31510 (15)	0.0569 (2)	0.51098 (18)	0.0378 (4)
H3A	0.2822	−0.0455	0.5334	0.062 (8)*
H3B	0.3536	0.0979	0.5908	0.043 (6)*
C4	0.20777 (10)	0.36153 (15)	0.29783 (14)	0.0224 (2)
C5	0.24386 (13)	0.44301 (17)	0.18458 (16)	0.0275 (3)
H5	0.3110	0.4142	0.1427	0.033 (5)*
C6	0.18230 (12)	0.56560 (17)	0.13300 (16)	0.0291 (3)
H6	0.2075	0.6201	0.0562	0.043 (6)*
C7	0.08318 (11)	0.60908 (16)	0.19388 (13)	0.0239 (2)
C8	0.04624 (11)	0.52707 (17)	0.30611 (16)	0.0269 (3)
H8	−0.0213	0.5551	0.3474	0.050 (7)*
C9	0.10815 (10)	0.40481 (17)	0.35732 (14)	0.0261 (3)
H9	0.0826	0.3498	0.4337	0.047 (6)*
C10	0.3736 (2)	−0.1097 (2)	0.3160 (2)	0.0525 (5)
H10A	0.2946	−0.1211	0.2992	0.074 (4)*
H10B	0.3997	−0.1986	0.3690	0.074 (4)*
H10C	0.4132	−0.1066	0.2307	0.074 (4)*
C11	0.51411 (15)	0.0563 (3)	0.4337 (3)	0.0559 (6)
H11A	0.5605	0.0584	0.3535	0.074 (4)*
H11B	0.5347	−0.0332	0.4894	0.074 (4)*
H11C	0.5246	0.1530	0.4844	0.074 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0301 (5)	0.0300 (5)	0.0304 (5)	0.0037 (4)	0.0062 (4)	0.0097 (4)
O2	0.0312 (5)	0.0299 (5)	0.0273 (5)	0.0088 (4)	0.0023 (4)	0.0043 (4)
N1	0.0234 (5)	0.0258 (5)	0.0279 (5)	0.0032 (4)	0.0001 (4)	0.0068 (4)
C1	0.0213 (5)	0.0223 (5)	0.0237 (6)	−0.0031 (4)	−0.0009 (4)	0.0022 (5)
C2	0.0283 (6)	0.0292 (7)	0.0292 (7)	0.0058 (5)	0.0021 (5)	0.0092 (5)
C3	0.0428 (8)	0.0388 (8)	0.0319 (7)	0.0138 (6)	0.0060 (7)	0.0132 (6)
C4	0.0216 (5)	0.0223 (5)	0.0233 (6)	−0.0007 (4)	−0.0002 (4)	0.0007 (5)
C5	0.0272 (6)	0.0290 (6)	0.0264 (6)	0.0058 (5)	0.0059 (5)	0.0040 (5)
C6	0.0309 (6)	0.0293 (6)	0.0272 (6)	0.0064 (5)	0.0070 (5)	0.0061 (6)
C7	0.0257 (5)	0.0229 (6)	0.0231 (6)	0.0017 (5)	−0.0003 (5)	0.0004 (5)
C8	0.0225 (5)	0.0308 (6)	0.0274 (6)	0.0029 (5)	0.0035 (5)	0.0037 (5)
C9	0.0234 (5)	0.0292 (6)	0.0256 (6)	−0.0005 (5)	0.0026 (5)	0.0049 (5)
C10	0.0859 (15)	0.0299 (8)	0.0418 (10)	0.0135 (9)	0.0005 (11)	0.0022 (8)
C11	0.0320 (8)	0.0674 (14)	0.0684 (15)	0.0008 (8)	−0.0094 (9)	0.0382 (12)

Geometric parameters (Å, º)

O1—C1	1.3640 (17)	C5—C6	1.389 (2)
O1—C3	1.4489 (18)	C5—H5	0.9500
O2—H2	0.8400	C6—C7	1.4007 (19)
N1—C1	1.2826 (17)	C6—H6	0.9500
N1—C2	1.4921 (18)	C7—C8	1.3981 (19)
C1—C4	1.4652 (18)	C8—C9	1.3878 (19)
O2—C7	1.3566 (16)	C8—H8	0.9500
C2—C3	1.531 (2)	C9—H9	0.9500
C2—C10	1.529 (3)	C10—H10A	0.9800
C2—C11	1.515 (2)	C10—H10B	0.9800
C3—H3A	0.9900	C10—H10C	0.9800
C3—H3B	0.9900	C11—H11A	0.9800
C4—C9	1.3994 (17)	C11—H11B	0.9800
C4—C5	1.4009 (19)	C11—H11C	0.9800
C1—O1—C3	105.69 (11)	C5—C6—C7	120.22 (13)
C7—O2—H2	109.5	C5—C6—H6	119.9
C1—N1—C2	107.59 (11)	C7—C6—H6	119.9
N1—C1—O1	117.06 (11)	O2—C7—C8	122.59 (12)
N1—C1—C4	127.30 (13)	O2—C7—C6	117.96 (12)
O1—C1—C4	115.64 (11)	C8—C7—C6	119.45 (12)
N1—C2—C3	102.55 (11)	C9—C8—C7	120.08 (12)
N1—C2—C10	108.60 (14)	C9—C8—H8	120.0
C3—C2—C10	110.80 (15)	C7—C8—H8	120.0
N1—C2—C11	110.09 (13)	C8—C9—C4	120.84 (13)
C3—C2—C11	113.00 (17)	C8—C9—H9	119.6
C10—C2—C11	111.37 (17)	C4—C9—H9	119.6
O1—C3—C2	104.96 (12)	C2—C10—H10A	109.5
O1—C3—H3A	110.8	C2—C10—H10B	109.5
C2—C3—H3A	110.8	H10A—C10—H10B	109.5
O1—C3—H3B	110.8	C2—C10—H10C	109.5
C2—C3—H3B	110.8	H10A—C10—H10C	109.5
H3A—C3—H3B	108.8	H10B—C10—H10C	109.5
C9—C4—C5	118.87 (12)	C2—C11—H11A	109.5
C9—C4—C1	120.36 (12)	C2—C11—H11B	109.5
C5—C4—C1	120.77 (12)	H11A—C11—H11B	109.5
C6—C5—C4	120.54 (13)	C2—C11—H11C	109.5
C6—C5—H5	119.7	H11A—C11—H11C	109.5
C4—C5—H5	119.7	H11B—C11—H11C	109.5
C2—N1—C1—O1	2.42 (17)	N1—C1—C4—C5	−2.2 (2)
C2—N1—C1—C4	−177.73 (13)	O1—C1—C4—C5	177.61 (13)
C3—O1—C1—N1	7.33 (18)	C9—C4—C5—C6	0.6 (2)
C3—O1—C1—C4	−172.54 (13)	C1—C4—C5—C6	−179.96 (14)
C1—N1—C2—C3	−10.43 (16)	C4—C5—C6—C7	0.0 (2)
C1—N1—C2—C10	106.88 (16)	C5—C6—C7—O2	179.74 (14)
C1—N1—C2—C11	−130.94 (17)	C5—C6—C7—C8	−0.6 (2)
C1—O1—C3—C2	−13.26 (17)	O2—C7—C8—C9	−179.68 (14)
N1—C2—C3—O1	14.24 (17)	C6—C7—C8—C9	0.7 (2)
C10—C2—C3—O1	−101.49 (16)	C7—C8—C9—C4	−0.1 (2)
C11—C2—C3—O1	132.72 (15)	C5—C4—C9—C8	−0.5 (2)
N1—C1—C4—C9	177.18 (14)	C1—C4—C9—C8	−179.96 (13)
O1—C1—C4—C9	−2.97 (19)