Structural characterization of p-benzosemiquinone radical in a solid state: the radical stabilization by a low-barrier hydrogen bond

Semiquinone (p-benzosemiquinone), a transient organic radical, was detected in the solid state by EPR spectroscopy revealing four symmetrically equivalent protons. A variable-temperature X-ray diffraction analysis (293 and 90 K) and EPR data support a dynamical disorder of the proton. A low-barrier O—H···O hydrogen bond stabilizes the radical. The C—O bond length is 1.297 (4) Å, corresponding to a bond order of ca 1.5. The geometry of the radical implies an electron delocalization throughout the benzenoid ring. Two polymorphs of semiquinone, monoclinic and triclinic, were observed and their structures determined. Their crystal packings were compared with those of quinhydrone polymorphs.

S1. Comment

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S2. Experimental

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S3. Refinement

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Computing details

For all compounds, data collection: CAD-4 EXPRESS (Enraf Nonius, 1994); cell refinement: CAD-4 EXPRESS (Enraf Nonius, 1994); data reduction: XCAD4 (Harms & Wocadlo, 1995). Program(s) used to solve structure: SHELXS97 (Sheldrick, 1997) for sqt90k, sqm293k, sqt293k; SHELXS86 (Sheldrick, 1986) for sqm90k. For all compounds, program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX publication routines (Farrugia, 1999).

Figures

Figure 1. View of (I) (50% probability displacement ellipsoids)

(sqt90k)

Crystal data

C6H5O2	Z = 1
Mr = 109.1	F(000) = 51
Triclinic, P1	Dx = 1.532 Mg m^−3
a = 3.7615 (6) Å	Cu Kα radiation, λ = 1.54179 Å
b = 5.6996 (12) Å	Cell parameters from 25 reflections
c = 5.9065 (8) Å	θ = 8.3–30°
α = 110.800 (15)°	µ = 0.97 mm^−1
β = 91.164 (12)°	T = 90 K
γ = 91.443 (15)°	Rod, red
V = 118.28 (4) Å^3	0.29 × 0.14 × 0.1 mm

Data collection

Enraf Nonius CAD4 diffractometer	θmax = 76.4°, θmin = 8.0°
non–profiled ω/2θ scans	h = −4→4
1075 measured reflections	k = −7→7
499 independent reflections	l = −7→7
422 reflections with I > 2σ(I)	3 standard reflections every 120 min
Rint = 0.056	intensity decay: 2%

Refinement

Refinement on F^2	All H-atom parameters refined
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0482P)^2 + 0.0384P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.036	(Δ/σ)max < 0.001
wR(F^2) = 0.106	Δρmax = 0.35 e Å^−3
S = 1.09	Δρmin = −0.2 e Å^−3
499 reflections	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
50 parameters	Extinction coefficient: 0.054 (15)
0 restraints

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.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
H1	0.275 (4)	0.076 (3)	0.359 (3)	0.016 (4)*
H2	0.641 (4)	0.216 (3)	0.101 (3)	0.018 (4)*
H3	0.046 (14)	0.459 (11)	0.931 (8)	0.050 (14)*	0.5
C1	0.3718 (3)	0.2476 (2)	0.4109 (2)	0.0136 (4)
O1	0.0852 (3)	0.32797 (18)	0.78144 (16)	0.0180 (4)
C3	0.2827 (3)	0.4133 (2)	0.6480 (2)	0.0126 (4)
C2	0.5838 (3)	0.3307 (2)	0.2693 (2)	0.0133 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0130 (7)	0.0126 (6)	0.0150 (7)	−0.0012 (5)	−0.0016 (5)	0.0050 (5)
O1	0.0238 (6)	0.0169 (6)	0.0139 (6)	−0.0017 (4)	0.0038 (4)	0.0062 (4)
C3	0.0113 (7)	0.0153 (7)	0.0126 (7)	−0.0010 (5)	0.0003 (5)	0.0065 (5)
C2	0.0130 (7)	0.0143 (7)	0.0115 (7)	0.0000 (5)	0.0004 (5)	0.0031 (5)

Geometric parameters (Å, º)

C1—C2	1.3599 (18)	O1—H3	0.95 (5)
C1—C3	1.4357 (19)	C3—C2^i	1.4378 (17)
C1—H1	0.972 (18)	C2—C3^i	1.4378 (17)
O1—C3	1.2988 (16)	C2—H2	1.007 (18)
C2—C1—C3	120.45 (11)	O1—C3—C2^i	121.98 (11)
C2—C1—H1	122.5 (10)	C1—C3—C2^i	118.78 (12)
C3—C1—H1	117.0 (10)	C1—C2—C3^i	120.76 (12)
C3—O1—H3	110 (4)	C1—C2—H2	120.4 (10)
O1—C3—C1	119.24 (11)	C3^i—C2—H2	118.8 (10)

Symmetry code: (i) −x+1, −y+1, −z+1.

(sqm293k)

Crystal data

C6H5O2	F(000) = 114
Mr = 109.1	Dx = 1.446 Mg m^−3
Monoclinic, P21/c	Cu Kα radiation, λ = 1.54179 Å
a = 3.8362 (2) Å	Cell parameters from 25 reflections
b = 5.9980 (3) Å	θ = 8.1–28.8°
c = 11.5007 (6) Å	µ = 0.92 mm^−1
β = 108.790 (1)°	T = 293 K
V = 250.52 (2) Å^3	Plate, green
Z = 2	0.25 × 0.15 × 0.1 mm

Data collection

Enraf Nonius CAD4 diffractometer	θmax = 76.0°, θmin = 8.1°
non–profiled ω/2θ scans	h = 0→4
601 measured reflections	k = 0→7
523 independent reflections	l = −14→13
412 reflections with I > 2σ(I)	3 standard reflections every 120 min
Rint = 0.066	intensity decay: 3%

Refinement

Refinement on F^2	All H-atom parameters refined
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0454P)^2 + 0.0735P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.039	(Δ/σ)max < 0.001
wR(F^2) = 0.106	Δρmax = 0.22 e Å^−3
S = 1.04	Δρmin = −0.15 e Å^−3
523 reflections	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
50 parameters	Extinction coefficient: 0.025 (7)
0 restraints

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.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
H3	−0.028 (5)	0.486 (3)	0.2924 (18)	0.041 (5)*
H2	0.305 (6)	0.216 (4)	0.639 (2)	0.052 (5)*
H1	0.421 (16)	0.067 (10)	0.474 (6)	0.09 (2)*	0.5
O1	0.3177 (4)	0.1598 (2)	0.41739 (13)	0.0512 (5)
C1	0.1673 (4)	0.3216 (3)	0.45904 (15)	0.0337 (4)
C3	−0.0196 (4)	0.4930 (3)	0.37616 (15)	0.0354 (4)
C2	0.1830 (4)	0.3351 (3)	0.58420 (15)	0.0354 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0616 (9)	0.0377 (8)	0.0502 (9)	0.0145 (7)	0.0124 (6)	−0.0086 (6)
C1	0.0360 (8)	0.0289 (8)	0.0373 (8)	0.0034 (7)	0.0135 (6)	−0.0033 (7)
C3	0.0400 (8)	0.0375 (9)	0.0296 (8)	0.0034 (7)	0.0127 (6)	0.0012 (7)
C2	0.0373 (8)	0.0306 (8)	0.0371 (9)	0.0049 (7)	0.0105 (6)	0.0057 (7)

Geometric parameters (Å, º)

O1—C1	1.297 (2)	C3—C2^i	1.359 (2)
O1—H1	0.85 (6)	C3—H3	0.955 (19)
C1—C2	1.424 (2)	C2—C3^i	1.359 (2)
C1—C3	1.428 (2)	C2—H2	0.97 (2)
C1—O1—H1	111 (4)	C2^i—C3—H3	120.1 (12)
O1—C1—C2	122.51 (16)	C1—C3—H3	119.3 (12)
O1—C1—C3	118.65 (15)	C3^i—C2—C1	120.58 (15)
C2—C1—C3	118.83 (14)	C3^i—C2—H2	121.1 (13)
C2^i—C3—C1	120.58 (14)	C1—C2—H2	118.2 (13)

Symmetry code: (i) −x, −y+1, −z+1.

(sqm90k)

Crystal data

C6H5O2	F(000) = 114
Mr = 109.1	Dx = 1.492 Mg m^−3
Monoclinic, P21/c	Cu Kα radiation, λ = 1.54184 Å
a = 3.770 (3) Å	Cell parameters from 16 reflections
b = 5.980 (3) Å	θ = 8.2–25.8°
c = 11.458 (14) Å	µ = 0.95 mm^−1
β = 109.99 (8)°	T = 90 K
V = 242.8 (4) Å^3	Plate, red
Z = 2	0.18 × 0.07 × 0.05 mm

Data collection

Enraf Nonius CAD4 diffractometer	θmax = 75.8°, θmin = 8.2°
non–profiled ω/2θ scans	h = −4→4
531 measured reflections	k = −7→0
507 independent reflections	l = −4→14
382 reflections with I > 2σ(I)	3 standard reflections every 120 min
Rint = 0.102	intensity decay: 2%

Refinement

Refinement on F^2	All H-atom parameters refined
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0849P)^2] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.047	(Δ/σ)max < 0.001
wR(F^2) = 0.131	Δρmax = 0.33 e Å^−3
S = 1.09	Δρmin = −0.42 e Å^−3
507 reflections	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
50 parameters	Extinction coefficient: 0.053 (13)
0 restraints

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.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
O1	0.5021 (4)	0.3395 (2)	0.41682 (13)	0.0242 (5)
C1	1.0156 (5)	−0.1659 (3)	0.41428 (16)	0.0169 (5)
C2	0.7654 (5)	0.0050 (3)	0.37502 (17)	0.0166 (5)
C3	0.7420 (5)	0.1783 (3)	0.45841 (17)	0.0162 (5)
H2	0.610 (6)	0.015 (4)	0.294 (2)	0.019 (5)*
H1	1.025 (7)	−0.283 (5)	0.357 (3)	0.029 (6)*
H3	0.498 (14)	0.429 (7)	0.478 (4)	0.018 (12)*	0.5

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0334 (9)	0.0161 (8)	0.0254 (8)	0.0090 (6)	0.0132 (6)	0.0041 (6)
C1	0.0185 (9)	0.0141 (9)	0.0197 (9)	−0.0012 (7)	0.0086 (7)	−0.0015 (7)
C2	0.0158 (8)	0.0175 (9)	0.0161 (8)	−0.0014 (7)	0.0051 (6)	−0.0021 (8)
C3	0.0144 (8)	0.0136 (8)	0.0206 (9)	0.0009 (7)	0.0061 (6)	0.0013 (7)

Geometric parameters (Å, º)

O1—C3	1.296 (2)	C1—H1	0.97 (3)
O1—H3	0.88 (4)	C2—C3	1.433 (3)
C1—C2	1.359 (3)	C2—H2	0.92 (2)
C1—C3^i	1.434 (3)	C3—C1^i	1.434 (3)
C3—O1—H3	111 (3)	C1—C2—H2	121.1 (15)
C2—C1—C3^i	120.29 (17)	C3—C2—H2	117.8 (16)
C2—C1—H1	120.1 (16)	O1—C3—C2	119.16 (18)
C3^i—C1—H1	119.6 (16)	O1—C3—C1^i	122.30 (18)
C1—C2—C3	121.15 (18)	C2—C3—C1^i	118.54 (18)

Symmetry code: (i) −x+2, −y, −z+1.

(sqt293k)

Crystal data

C6H5O2	Z = 1
Mr = 109.1	F(000) = 51
Triclinic, P1	Dx = 1.476 Mg m^−3
a = 3.8291 (2) Å	Cu Kα radiation, λ = 1.54179 Å
b = 5.7546 (3) Å	Cell parameters from 25 reflections
c = 5.9771 (2) Å	θ = 11.6–27.4°
α = 111.121 (3)°	µ = 0.94 mm^−1
β = 89.936 (4)°	T = 293 K
γ = 92.453 (4)°	Rod, red
V = 122.73 (1) Å^3	0.28 × 0.14 × 0.1 mm

Data collection

Enraf Nonius CAD4 diffractometer	θmax = 76.2°, θmin = 8.0°
non–profiled ω/2θ scans	h = −4→4
654 measured reflections	k = −6→7
513 independent reflections	l = −7→0
409 reflections with I > 2σ(I)	3 standard reflections every 120 min
Rint = 0.062	intensity decay: 10%

Refinement

Refinement on F^2	All H-atom parameters refined
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0702P)^2 + 0.0343P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.043	(Δ/σ)max < 0.001
wR(F^2) = 0.124	Δρmax = 0.24 e Å^−3
S = 1.04	Δρmin = −0.25 e Å^−3
513 reflections	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
50 parameters	Extinction coefficient: 0.06 (2)
0 restraints

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.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
H2	0.283 (5)	0.081 (4)	0.367 (4)	0.039 (5)*
H3	0.636 (6)	0.221 (4)	0.104 (4)	0.047 (6)*
H1	0.057 (13)	0.458 (10)	0.926 (8)	0.051 (13)*	0.5
C2	0.3722 (4)	0.2500 (3)	0.4123 (3)	0.0326 (5)
O1	0.0890 (4)	0.3286 (2)	0.7807 (2)	0.0443 (5)
C1	0.2853 (4)	0.4142 (3)	0.6475 (3)	0.0303 (5)
C3	0.5823 (4)	0.3332 (3)	0.2716 (3)	0.0332 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C2	0.0360 (9)	0.0285 (8)	0.0311 (9)	−0.0050 (6)	0.0030 (6)	0.0089 (7)
O1	0.0562 (9)	0.0428 (8)	0.0340 (8)	−0.0080 (6)	0.0146 (6)	0.0153 (6)
C1	0.0306 (8)	0.0337 (9)	0.0279 (8)	−0.0023 (6)	0.0061 (6)	0.0129 (7)
C3	0.0368 (9)	0.0335 (9)	0.0266 (8)	−0.0016 (7)	0.0069 (7)	0.0078 (7)

Geometric parameters (Å, º)

C2—C3	1.356 (2)	O1—H1	0.93 (5)
C2—C1	1.431 (2)	C1—C3^i	1.426 (2)
C2—H2	0.96 (2)	C3—C1^i	1.426 (2)
O1—C1	1.3004 (19)	C3—H3	1.00 (2)
C3—C2—C1	120.25 (15)	O1—C1—C2	119.06 (14)
C3—C2—H2	123.6 (13)	C3^i—C1—C2	118.84 (14)
C1—C2—H2	116.0 (13)	C2—C3—C1^i	120.90 (14)
C1—O1—H1	108 (3)	C2—C3—H3	120.8 (13)
O1—C1—C3^i	122.10 (14)	C1^i—C3—H3	118.2 (12)

Symmetry code: (i) −x+1, −y+1, −z+1.