3,4-Dinitro-2,5-bis­[4-(trifluoro­meth­yl)phen­yl]thio­phene

Huang, P.-H.; Shen, J.-Y.; Wen, Y.-S.

doi:10.1107/S1600536809020443

organic compounds

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 65| Part 7| July 2009| Page o1469

doi:10.1107/S1600536809020443

Open

access

3,4-Dinitro-2,5-bis[4-(trifluoromethyl)phenyl]thiophene

Ping-Hsin Huang,^a ^* Jiun-Yi Shen ^b and Yuh-Sheng Wen ^b

^aCardinal Tien College of Healthcare and Management, Taipei 231, Taiwan, and ^bInstitute of Chemistry, Academia Sinica, Nankang, Taipei, Taiwan
^*Correspondence e-mail: pshuang@ctcn.edu.tw

(Received 21 May 2009; accepted 28 May 2009; online 6 June 2009)

The title compound, C₁₈H₈F₆N₂O₄S, is a precursor for the production of low-band-gap conjugated polymers. In the crystal structure, the dihedral angles between the thiophene and benzene rings are 35.90 (8) and 61.94 (8)°, and that between the two benzene rings is 40.18 (8)°. The two nitro groups are twisted with respect to the thiophene ring, the dihedral angles being 53.66 (10) and 31.63 (10)°. Weak intermolecular C—H⋯O hydrogen bonding helps to stabilize the crystal structure.

Related literature

For a related structure, see: Bak et al. (1961 ).

Experimental

Crystal data

C₁₈H₈F₆N₂O₄S
M_r = 462.32
Orthorhombic, P b c a
a = 8.1572 (3) Å
b = 17.6371 (6) Å
c = 24.4150 (8) Å
V = 3512.6 (2) Å³
Z = 8
Mo Kα radiation
μ = 0.28 mm⁻¹
T = 100 K
0.4 × 0.36 × 0.1 mm

Data collection

Bruker SMART CCD area-detector diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2001 ) T_min = 0.895, T_max = 0.973
22650 measured reflections
3098 independent reflections
1888 reflections with I > 2σ(I)
R_int = 0.058

Refinement

R[F² > 2σ(F²)] = 0.030
wR(F²) = 0.053
S = 0.80
3098 reflections
281 parameters
H-atom parameters constrained
Δρ_max = 0.29 e Å⁻³
Δρ_min = −0.28 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C13—H13⋯O4ⁱ	0.93	2.52	3.320 (2)	144
Symmetry code: (i) $[-x+1, y+{\script{1\over 2}}, -z+{\script{1\over 2}}]$ .

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997 ); software used to prepare material for publication: WinGX (Farrugia, 1999 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536809020443/xu2528sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536809020443/xu2528Isup2.hkl

checkCIF report

Comment top

The title compound, (I), has been shown to be an excellent precursor for the production of low band gap conjugated polymers and organic light-emitting devices etc. As indicated in Scheme 2, standard procedures were administrated to synthesize in high yield. The molecular structure is shown in Fig. 1. The double bonds and C—C single bond of (I) are slightly shorter than those of the parent thiophene, while the S—C single bond is slightly elongated (Bak et al., 1961). The dihedral angles between the thiophene (S/C1–C4) and benzene rings (C11–C16 and C21–C26) are 35.90 (8) and 61.94 (8)°, respectively, and that between the two benzene rings is 40.18 (8)°. The two nitro groups are oriented at the thiophene ring with the dihedral angles of 53.66 (10) and 31.63 (10)°, respectively. Intermolecular weak C—H···O hydrogen bonding helps to stabilize the crystal structure (Table 1).

Related literature top

For a related structure, see: Bak et al. (1961).

Experimental top

The compound was synthesized by the following procedure. A two-necked round-bottomed flask was charged with Pd(PPh₃)₄ (280 mg), tributyl(4-(trifluoromethyl)phenyl)stannane (3.26 g, 7.5 mmol), 2,5-dibromo-3,4-dinitrothiophene (1.00 g, 3.0 mmol) and DMF (20 ml), and the reaction mixture stirred under nitrogen and heated at 343 K for 48 h. After cooling, the mixture was diluted with diethyl ether and the organic phase was washed with water and brine. After drying over anhydrous MgSO₄ and removing the volatiles, the residue was purified by column chromatography using CH₂Cl₂/n-hexane as eluent, followed by recrystallization from CH₂Cl₂ and hexane to yield 0.7 g (50%) of (I) as a white solid. Crystals suitable for X-ray diffraction were grown from a CH₂Cl₂ solution layered with hexane at room temperature.

Computing details top

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT (Bruker, 2001; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures top

	Fig. 1. A molecular structure of (I) with 30% probability displacement ellipsoids, showing the atom-numbering scheme employed. H atoms are shown as small spheres of the arbitrary radii.
	Fig. 2. The formation of the title compound.

3,4-Dinitro-2,5-bis[4-(trifluoromethyl)phenyl]thiophene top

Crystal data top

C₁₈H₈F₆N₂O₄S	F(000) = 1856
M_r = 462.32	D_x = 1.748 Mg m⁻³
Orthorhombic, Pbca	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2ab	Cell parameters from 3689 reflections
a = 8.1572 (3) Å	θ = 2.3–21.2°
b = 17.6371 (6) Å	µ = 0.28 mm⁻¹
c = 24.4150 (8) Å	T = 100 K
V = 3512.6 (2) Å³	Plate, colourless
Z = 8	0.4 × 0.36 × 0.1 mm

Data collection top

Bruker SMART CCD area-detector diffractometer	3098 independent reflections
Radiation source: fine-focus sealed tube	1888 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.058
ω and ϕ scans	θ_max = 25.0°, θ_min = 1.7°
Absorption correction: multi-scan (SADABS; Bruker, 2001)	h = −8→9
T_min = 0.895, T_max = 0.973	k = −20→20
22650 measured reflections	l = −29→29

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.030	H-atom parameters constrained
wR(F²) = 0.053	w = 1/[σ²(F_o²) + (0.0222P)²] where P = (F_o² + 2F_c²)/3
S = 0.80	(Δ/σ)_max < 0.001
3098 reflections	Δρ_max = 0.29 e Å⁻³
281 parameters	Δρ_min = −0.28 e Å⁻³
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.00042 (7)

Crystal data top

C₁₈H₈F₆N₂O₄S	V = 3512.6 (2) Å³
M_r = 462.32	Z = 8
Orthorhombic, Pbca	Mo Kα radiation
a = 8.1572 (3) Å	µ = 0.28 mm⁻¹
b = 17.6371 (6) Å	T = 100 K
c = 24.4150 (8) Å	0.4 × 0.36 × 0.1 mm

Data collection top

Bruker SMART CCD area-detector diffractometer	3098 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2001)	1888 reflections with I > 2σ(I)
T_min = 0.895, T_max = 0.973	R_int = 0.058
22650 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.030	0 restraints
wR(F²) = 0.053	H-atom parameters constrained
S = 0.80	Δρ_max = 0.29 e Å⁻³
3098 reflections	Δρ_min = −0.28 e Å⁻³
281 parameters

Special details top

Experimental. ¹H NMR (CDCl₃): 7.77 (d, J = 8.2, 4H), 7.64 (d, J = 8.2, 4H). FAB MS (m/e): 462 (M⁺). Analysis calculated for C₁₈H₈F₆N₂O₄S: C 46.76, H 1.74, N 6.06%; found: C 46.80, H 1.88, N 5.79%.

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 (Å²) top

	x	y	z	U_iso*/U_eq
S	0.49028 (6)	0.16607 (3)	0.10370 (2)	0.01968 (14)
F1	0.35605 (13)	0.58240 (6)	0.14011 (4)	0.0308 (3)
F2	0.55441 (13)	0.58514 (6)	0.08266 (5)	0.0355 (3)
F3	0.31448 (13)	0.55311 (6)	0.05612 (4)	0.0317 (3)
F4	0.39559 (15)	−0.25593 (7)	0.10705 (5)	0.0436 (4)
F5	0.59498 (14)	−0.24370 (7)	0.05129 (5)	0.0414 (4)
F6	0.35364 (14)	−0.21365 (7)	0.02679 (5)	0.0421 (4)
O1	0.84762 (17)	0.29479 (8)	0.21049 (6)	0.0336 (4)
O2	0.72134 (16)	0.23172 (8)	0.27348 (6)	0.0327 (4)
O3	0.89423 (17)	0.10570 (8)	0.23361 (6)	0.0326 (4)
O4	0.72044 (15)	0.01316 (8)	0.23007 (5)	0.0227 (4)
N2	0.75230 (19)	0.24606 (10)	0.22593 (7)	0.0206 (4)
N3	0.7648 (2)	0.07741 (10)	0.21850 (6)	0.0198 (4)
C1	0.5770 (2)	0.23578 (11)	0.14335 (7)	0.0154 (5)
C2	0.6654 (2)	0.20278 (11)	0.18389 (8)	0.0152 (5)
C3	0.6626 (2)	0.12295 (11)	0.18302 (7)	0.0151 (5)
C4	0.5731 (2)	0.09335 (10)	0.14100 (7)	0.0145 (5)
C11	0.5395 (2)	0.31580 (10)	0.13231 (8)	0.0153 (5)
C12	0.5170 (2)	0.36726 (11)	0.17431 (8)	0.0177 (5)
H12	0.5278	0.3514	0.2105	0.021*
C13	0.4788 (2)	0.44149 (11)	0.16326 (8)	0.0182 (5)
H13	0.4654	0.4758	0.1918	0.022*
C14	0.4601 (2)	0.46509 (11)	0.10993 (8)	0.0169 (5)
C15	0.4800 (2)	0.41410 (11)	0.06754 (8)	0.0213 (5)
H15	0.4659	0.4299	0.0315	0.026*
C16	0.5204 (2)	0.34036 (11)	0.07853 (8)	0.0203 (5)
H16	0.5353	0.3064	0.0498	0.024*
C17	0.4211 (2)	0.54560 (12)	0.09785 (9)	0.0225 (5)
C21	0.5441 (2)	0.01459 (11)	0.12329 (8)	0.0154 (5)
C22	0.5951 (2)	−0.00879 (11)	0.07186 (8)	0.0191 (5)
H22	0.6485	0.0252	0.0488	0.023*
C23	0.5672 (2)	−0.08169 (11)	0.05477 (8)	0.0204 (5)
H23	0.6030	−0.0973	0.0204	0.024*
C24	0.4859 (2)	−0.13201 (11)	0.08861 (7)	0.0167 (5)
C25	0.4314 (2)	−0.10885 (11)	0.13930 (8)	0.0182 (5)
H25	0.3749	−0.1424	0.1618	0.022*
C26	0.4607 (2)	−0.03585 (11)	0.15652 (8)	0.0171 (5)
H26	0.4242	−0.0203	0.1908	0.021*
C27	0.4586 (3)	−0.21090 (12)	0.06936 (9)	0.0249 (5)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
S	0.0205 (3)	0.0148 (3)	0.0238 (3)	−0.0007 (3)	−0.0048 (3)	0.0006 (2)
F1	0.0375 (8)	0.0209 (7)	0.0339 (8)	0.0100 (6)	0.0032 (6)	−0.0016 (6)
F2	0.0225 (7)	0.0184 (7)	0.0654 (9)	−0.0028 (6)	0.0090 (6)	0.0108 (6)
F3	0.0315 (7)	0.0291 (8)	0.0345 (8)	0.0085 (6)	−0.0084 (6)	0.0057 (6)
F4	0.0709 (10)	0.0208 (7)	0.0391 (8)	−0.0164 (7)	0.0180 (7)	−0.0038 (6)
F5	0.0265 (8)	0.0240 (7)	0.0738 (10)	0.0025 (6)	0.0122 (7)	−0.0171 (7)
F6	0.0447 (8)	0.0355 (8)	0.0462 (9)	−0.0044 (6)	−0.0134 (7)	−0.0163 (7)
O1	0.0313 (9)	0.0344 (10)	0.0351 (9)	−0.0202 (8)	−0.0052 (8)	0.0047 (8)
O2	0.0415 (10)	0.0389 (10)	0.0176 (9)	−0.0115 (8)	−0.0001 (8)	−0.0018 (8)
O3	0.0219 (9)	0.0289 (9)	0.0469 (10)	−0.0039 (7)	−0.0169 (8)	−0.0003 (7)
O4	0.0227 (9)	0.0182 (8)	0.0271 (9)	−0.0001 (7)	0.0004 (7)	0.0057 (7)
N2	0.0183 (11)	0.0192 (11)	0.0244 (12)	0.0025 (9)	−0.0038 (9)	−0.0018 (9)
N3	0.0189 (11)	0.0199 (11)	0.0206 (10)	0.0016 (9)	0.0001 (9)	−0.0022 (9)
C1	0.0117 (11)	0.0178 (12)	0.0167 (11)	−0.0027 (9)	0.0006 (9)	−0.0006 (10)
C2	0.0112 (11)	0.0169 (12)	0.0177 (12)	−0.0046 (10)	0.0003 (9)	−0.0032 (10)
C3	0.0109 (12)	0.0179 (12)	0.0166 (12)	0.0014 (10)	0.0006 (10)	0.0029 (10)
C4	0.0093 (11)	0.0180 (12)	0.0163 (11)	0.0018 (9)	0.0024 (9)	0.0021 (10)
C11	0.0083 (11)	0.0163 (12)	0.0213 (12)	−0.0042 (9)	0.0005 (10)	0.0001 (10)
C12	0.0155 (12)	0.0197 (12)	0.0179 (11)	−0.0020 (10)	−0.0018 (10)	0.0029 (10)
C13	0.0146 (12)	0.0175 (12)	0.0226 (12)	−0.0021 (10)	0.0025 (10)	−0.0028 (10)
C14	0.0101 (11)	0.0158 (12)	0.0248 (12)	−0.0031 (9)	−0.0007 (10)	0.0020 (10)
C15	0.0216 (12)	0.0206 (13)	0.0217 (12)	−0.0025 (11)	−0.0021 (10)	0.0030 (10)
C16	0.0214 (12)	0.0175 (12)	0.0220 (12)	−0.0015 (11)	0.0013 (10)	−0.0032 (10)
C17	0.0189 (13)	0.0217 (13)	0.0268 (14)	0.0010 (11)	0.0009 (11)	−0.0012 (11)
C21	0.0106 (12)	0.0166 (12)	0.0190 (12)	0.0024 (9)	−0.0041 (9)	0.0000 (9)
C22	0.0172 (12)	0.0196 (13)	0.0205 (12)	−0.0027 (10)	0.0001 (10)	0.0036 (10)
C23	0.0193 (12)	0.0240 (13)	0.0179 (12)	−0.0014 (10)	0.0025 (10)	−0.0039 (10)
C24	0.0109 (11)	0.0160 (11)	0.0233 (12)	0.0002 (10)	−0.0014 (10)	−0.0021 (9)
C25	0.0131 (12)	0.0182 (12)	0.0232 (12)	0.0006 (9)	0.0016 (10)	0.0045 (10)
C26	0.0151 (12)	0.0181 (12)	0.0182 (11)	0.0039 (10)	0.0015 (10)	−0.0002 (9)
C27	0.0233 (14)	0.0249 (13)	0.0266 (13)	−0.0003 (11)	0.0048 (12)	−0.0020 (11)

Geometric parameters (Å, º) top

S—C4	1.7119 (19)	C12—C13	1.373 (2)
S—C1	1.7173 (19)	C12—H12	0.9300
F1—C17	1.329 (2)	C13—C14	1.375 (2)
F2—C17	1.344 (2)	C13—H13	0.9300
F3—C17	1.346 (2)	C14—C15	1.381 (2)
F4—C27	1.320 (2)	C14—C17	1.485 (3)
F5—C27	1.330 (2)	C15—C16	1.368 (3)
F6—C27	1.347 (2)	C15—H15	0.9300
O1—N2	1.2187 (19)	C16—H16	0.9300
O2—N2	1.2147 (18)	C21—C26	1.383 (2)
O3—N3	1.2249 (18)	C21—C22	1.386 (2)
O4—N3	1.2226 (18)	C22—C23	1.371 (3)
N2—C2	1.462 (2)	C22—H22	0.9300
N3—C3	1.446 (2)	C23—C24	1.382 (3)
C1—C2	1.356 (2)	C23—H23	0.9300
C1—C11	1.469 (2)	C24—C25	1.377 (2)
C2—C3	1.408 (2)	C24—C27	1.485 (3)
C3—C4	1.363 (2)	C25—C26	1.375 (2)
C4—C21	1.474 (2)	C25—H25	0.9300
C11—C12	1.382 (2)	C26—H26	0.9300
C11—C16	1.391 (2)

C4—S—C1	94.24 (9)	C15—C16—C11	120.54 (18)
O2—N2—O1	125.10 (17)	C15—C16—H16	119.7
O2—N2—C2	117.48 (17)	C11—C16—H16	119.7
O1—N2—C2	117.40 (17)	F1—C17—F2	106.49 (16)
O4—N3—O3	124.26 (17)	F1—C17—F3	106.33 (16)
O4—N3—C3	118.87 (17)	F2—C17—F3	105.24 (16)
O3—N3—C3	116.82 (17)	F1—C17—C14	113.47 (17)
C2—C1—C11	131.07 (18)	F2—C17—C14	112.20 (16)
C2—C1—S	108.86 (14)	F3—C17—C14	112.51 (17)
C11—C1—S	119.92 (14)	C26—C21—C22	119.21 (18)
C1—C2—C3	114.15 (17)	C26—C21—C4	120.86 (17)
C1—C2—N2	123.10 (18)	C22—C21—C4	119.88 (17)
C3—C2—N2	122.72 (18)	C23—C22—C21	120.34 (19)
C4—C3—C2	113.78 (17)	C23—C22—H22	119.8
C4—C3—N3	123.16 (18)	C21—C22—H22	119.8
C2—C3—N3	122.46 (18)	C22—C23—C24	120.00 (18)
C3—C4—C21	131.90 (18)	C22—C23—H23	120.0
C3—C4—S	108.95 (14)	C24—C23—H23	120.0
C21—C4—S	119.11 (14)	C25—C24—C23	120.11 (18)
C12—C11—C16	118.74 (18)	C25—C24—C27	120.92 (18)
C12—C11—C1	121.51 (17)	C23—C24—C27	118.97 (18)
C16—C11—C1	119.71 (17)	C26—C25—C24	119.76 (18)
C13—C12—C11	120.72 (17)	C26—C25—H25	120.1
C13—C12—H12	119.6	C24—C25—H25	120.1
C11—C12—H12	119.6	C25—C26—C21	120.55 (18)
C12—C13—C14	119.99 (18)	C25—C26—H26	119.7
C12—C13—H13	120.0	C21—C26—H26	119.7
C14—C13—H13	120.0	F4—C27—F5	107.18 (18)
C13—C14—C15	119.97 (19)	F4—C27—F6	105.61 (16)
C13—C14—C17	120.08 (18)	F5—C27—F6	105.08 (16)
C15—C14—C17	119.94 (18)	F4—C27—C24	113.67 (17)
C16—C15—C14	120.02 (18)	F5—C27—C24	112.76 (17)
C16—C15—H15	120.0	F6—C27—C24	111.91 (18)
C14—C15—H15	120.0

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C13—H13···O4ⁱ	0.93	2.52	3.320 (2)	144

Symmetry code: (i) −x+1, y+1/2, −z+1/2.

Experimental details

Crystal data
Chemical formula	C₁₈H₈F₆N₂O₄S
M_r	462.32
Crystal system, space group	Orthorhombic, Pbca
Temperature (K)	100
a, b, c (Å)	8.1572 (3), 17.6371 (6), 24.4150 (8)
V (Å³)	3512.6 (2)
Z	8
Radiation type	Mo Kα
µ (mm⁻¹)	0.28
Crystal size (mm)	0.4 × 0.36 × 0.1

Data collection
Diffractometer	Bruker SMART CCD area-detector diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2001)
T_min, T_max	0.895, 0.973
No. of measured, independent and observed [I > 2σ(I)] reflections	22650, 3098, 1888
R_int	0.058
(sin θ/λ)_max (Å⁻¹)	0.595

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.030, 0.053, 0.80
No. of reflections	3098
No. of parameters	281
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.29, −0.28

Computer programs: SMART (Bruker, 2001), SAINT (Bruker, 2001), SAINT (Bruker, 2001, SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), ORTEP-3 for Windows (Farrugia, 1997), WinGX (Farrugia, 1999).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C13—H13···O4ⁱ	0.93	2.52	3.320 (2)	144

Symmetry code: (i) −x+1, y+1/2, −z+1/2.

Acknowledgements

This work was partially supported by the Institute of Chemistry, Academia Sinica, and Cardinal Tien College of Healthcare and Management.

References

Bak, B., Christensen, D., Hansen-Nygaard, L. & Rastrup-Andersen, J. (1961). J. Mol. Spectrosc. 7, 58–63. CrossRef CAS Web of Science Google Scholar
Bruker (2001). SMART, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565. CrossRef IUCr Journals Google Scholar
Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837–838. CrossRef CAS IUCr Journals Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar