4,5-Diphenyl-2-p-tolyl-1H-imidazol-3-ium nitrate

Cui, L.-J.

doi:10.1107/S1600536809018789

organic compounds

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 65| Part 6| June 2009| Page o1361

doi:10.1107/S1600536809018789

Open

access

4,5-Diphenyl-2-p-tolyl-1H-imidazol-3-ium nitrate

Li-Jing Cui ^a ^*

^aOrdered Matter Science Research Center, College of Chemistry and Chemical Engineering, Southeast University, Nanjing 210096, People's Republic of China
^*Correspondence e-mail: fudavid88@yahoo.com.cn

(Received 14 April 2009; accepted 18 May 2009; online 23 May 2009)

In the cation of the title compound, C₂₂H₁₉N₂⁺·NO₃⁻, the N atom in the 3-position of the imidazole is protonated. The three pendant aromatic rings are twisted from the plane of the imadazolium ring by dihedral angles of 38.1 (1), 43.74 (9) and 20.4 (1)°. In the crystal structure, N—H⋯O and N—H⋯(O,O) hydrogen bonds link the molecules to form an infinite one-dimensional chain parallel to the c axis.

Related literature

For uses of imidazole derivatives, see: Dai & Fu (2008 ); Fu & Xiong (2008 ); Huang et al. (2008 ).

Experimental

Crystal data

C₂₂H₁₉N₂⁺·NO₃⁻
M_r = 373.40
Orthorhombic, P 2₁ 2₁ 2₁
a = 8.442 (2) Å
b = 12.970 (3) Å
c = 17.098 (3) Å
V = 1872.3 (6) Å³
Z = 4
Mo Kα radiation
μ = 0.09 mm⁻¹
T = 298 K
0.40 × 0.35 × 0.25 mm

Data collection

Rigaku Mercury2 diffractometer
Absorption correction: multi-scan (CrystalClear; Rigaku, 2005 ) T_min = 0.955, T_max = 1.000 (expected range = 0.934–0.978)
19745 measured reflections
4278 independent reflections
2714 reflections with I > 2σ(I)
R_int = 0.089

Refinement

R[F² > 2σ(F²)] = 0.059
wR(F²) = 0.143
S = 1.03
4278 reflections
254 parameters
H-atom parameters constrained
Δρ_max = 0.26 e Å⁻³
Δρ_min = −0.20 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N2—H2A⋯O1ⁱ	0.86	2.05	2.905 (4)	176
N2—H2A⋯O2ⁱ	0.86	2.39	2.922 (3)	121
N1—H1A⋯O3	0.86	1.96	2.720 (3)	147
Symmetry code: (i) $[-x+{\script{1\over 2}}, -y+1, z-{\script{1\over 2}}]$ .

Data collection: CrystalClear (Rigaku, 2005); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL/PC (Sheldrick, 2008); software used to prepare material for publication: SHELXTL/PC.

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536809018789/im2112sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536809018789/im2112Isup2.hkl

checkCIF report

Comment top

Imidazole derivatives have found wide range of applications in coordination chemistry because of their multiple coordination modes as ligands to metal ions and for the construction of novel metal–organic frameworks (Huang et al., 2008; Fu & Xiong, 2008; Dai & Fu, 2008). We report herein the crystal structure of the title compound, 4,5-diphenyl-2-p-tolyl-1H-imidazol-3-ium nitrate.

The title compound contains an organic cation and a nitrate ion in the asymmetric unit. The imidazole N atom in 3-position is protonated. Imidazole and benzene rings are twisted from each other by a dihedral angle of 38.1 (1)°, 43.74 (9)° and 20.4 (1)°. The crystal packing is stabilized by N—H···O hydrogen bonds to form an infinite one-dimensional chain parallel to c axis. (Table 1, Fig. 2).

Related literature top

For uses of imidazole derivatives, see: Dai & Fu (2008); Fu & Xiong (2008); Huang et al. (2008).

Experimental top

Under nitrogen protection, 1,2-diphenyl-ethane-1,2-dione (20 mmol), 4-methylbenzaldehyde (20 mmol) and amine acetate (50 mmol) were dissolved in 60 ml of HOAc. The mixture was stirred at 110 °C for 20 h. The resulting solution was poured into ice water (200 ml) and after neutralizing the mixture with NaOH (6 mol/l) a white solid was obtained. After filtration and washing with distilled water the crude product was recrystallized from an ethanolic solution (150 ml) to which nitric acid (5 ml) was added to yield colorless block-like crystals of the title compound, suitable for X-ray analysis.

Computing details top

Data collection: CrystalClear (Rigaku, 2005); cell refinement: CrystalClear (Rigaku, 2005); data reduction: CrystalClear (Rigaku, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL/PC (Sheldrick, 2008); software used to prepare material for publication: SHELXTL/PC (Sheldrick, 2008).

Figures top

	Fig. 1. A view of the title compound with the atomic numbering scheme. Displacement ellipsoids were drawn at the 30% probability level.
	Fig. 2. The crystal packing of the title compound viewed along the b axis showing the infinte chain realized by hydrogen bonds (dashed lines). H atoms not involved in hydrogen bonding have been omitted for clarity.

4,5-Diphenyl-2-p-tolyl-1H-imidazol-3-ium nitrate top

Crystal data top

C₂₂H₁₉N₂⁺·NO₃⁻	F(000) = 784
M_r = 373.40	D_x = 1.325 Mg m⁻³
Orthorhombic, P2₁2₁2₁	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2ab	Cell parameters from 2714 reflections
a = 8.442 (2) Å	θ = 3.1–27.5°
b = 12.970 (3) Å	µ = 0.09 mm⁻¹
c = 17.098 (3) Å	T = 298 K
V = 1872.3 (6) Å³	Block, colorless
Z = 4	0.40 × 0.35 × 0.25 mm

Data collection top

Rigaku Mercury2 diffractometer	4278 independent reflections
Radiation source: fine-focus sealed tube	2714 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.089
Detector resolution: 13.6612 pixels mm^-1	θ_max = 27.5°, θ_min = 3.1°
CCD profile fitting scans	h = −10→10
Absorption correction: multi-scan (CrystalClear; Rigaku, 2005)	k = −16→16
T_min = 0.955, T_max = 1.000	l = −22→22
19745 measured reflections

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.059	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.143	H-atom parameters constrained
S = 1.03	w = 1/[σ²(F_o²) + (0.0541P)² + 0.288P] where P = (F_o² + 2F_c²)/3
4278 reflections	(Δ/σ)_max < 0.001
254 parameters	Δρ_max = 0.26 e Å⁻³
0 restraints	Δρ_min = −0.20 e Å⁻³

Crystal data top

C₂₂H₁₉N₂⁺·NO₃⁻	V = 1872.3 (6) Å³
M_r = 373.40	Z = 4
Orthorhombic, P2₁2₁2₁	Mo Kα radiation
a = 8.442 (2) Å	µ = 0.09 mm⁻¹
b = 12.970 (3) Å	T = 298 K
c = 17.098 (3) Å	0.40 × 0.35 × 0.25 mm

Data collection top

Rigaku Mercury2 diffractometer	4278 independent reflections
Absorption correction: multi-scan (CrystalClear; Rigaku, 2005)	2714 reflections with I > 2σ(I)
T_min = 0.955, T_max = 1.000	R_int = 0.089
19745 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.059	0 restraints
wR(F²) = 0.143	H-atom parameters constrained
S = 1.03	Δρ_max = 0.26 e Å⁻³
4278 reflections	Δρ_min = −0.20 e Å⁻³
254 parameters

Special details top

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
N1	0.3099 (3)	0.44418 (17)	0.66981 (13)	0.0415 (6)
H1A	0.2804	0.4503	0.7177	0.050*
N2	0.3114 (3)	0.45286 (18)	0.54570 (13)	0.0438 (6)
H2A	0.2827	0.4654	0.4984	0.053*
C16	0.0757 (3)	0.5305 (2)	0.60980 (17)	0.0409 (7)
C8	0.4517 (3)	0.4054 (2)	0.56608 (16)	0.0402 (7)
C19	−0.2150 (3)	0.6340 (2)	0.61119 (19)	0.0464 (7)
C7	0.4501 (3)	0.3994 (2)	0.64587 (15)	0.0368 (6)
C15	0.2268 (3)	0.4767 (2)	0.60831 (17)	0.0407 (7)
C6	0.5633 (3)	0.3576 (2)	0.70270 (15)	0.0389 (7)
C1	0.6475 (3)	0.2682 (2)	0.68690 (17)	0.0462 (7)
H1	0.6301	0.2329	0.6403	0.055*
C21	0.0283 (4)	0.5898 (2)	0.54664 (17)	0.0482 (7)
H21	0.0938	0.5955	0.5031	0.058*
C10	0.7240 (4)	0.3933 (2)	0.51284 (18)	0.0492 (8)
H10	0.7630	0.4213	0.5591	0.059*
C17	−0.0230 (3)	0.5248 (2)	0.67441 (17)	0.0483 (8)
H17	0.0068	0.4859	0.7177	0.058*
C9	0.5648 (3)	0.3741 (2)	0.50526 (16)	0.0406 (7)
C12	0.7709 (4)	0.3280 (3)	0.38461 (18)	0.0559 (8)
H12	0.8403	0.3138	0.3438	0.067*
C20	−0.1144 (4)	0.6400 (2)	0.54785 (18)	0.0499 (8)
H20	−0.1442	0.6793	0.5048	0.060*
C14	0.5107 (4)	0.3290 (3)	0.43701 (17)	0.0550 (8)
H14	0.4035	0.3142	0.4315	0.066*
C13	0.6136 (4)	0.3057 (3)	0.37721 (18)	0.0611 (9)
H13	0.5761	0.2749	0.3318	0.073*
C4	0.6997 (4)	0.3718 (3)	0.82531 (18)	0.0594 (9)
H4	0.7178	0.4068	0.8719	0.071*
C11	0.8262 (4)	0.3713 (3)	0.45231 (19)	0.0577 (9)
H11	0.9335	0.3860	0.4575	0.069*
C18	−0.1656 (3)	0.5769 (2)	0.67453 (17)	0.0475 (8)
H18	−0.2300	0.5733	0.7186	0.057*
C5	0.5878 (4)	0.4079 (2)	0.77306 (16)	0.0482 (8)
H5	0.5287	0.4662	0.7852	0.058*
C2	0.7562 (4)	0.2313 (3)	0.73960 (19)	0.0583 (9)
H2	0.8112	0.1708	0.7290	0.070*
C3	0.7842 (4)	0.2839 (3)	0.8083 (2)	0.0635 (9)
H3	0.8603	0.2600	0.8431	0.076*
C22	−0.3743 (4)	0.6851 (3)	0.6105 (2)	0.0662 (9)
H22A	−0.4143	0.6869	0.5580	0.099*
H22B	−0.3645	0.7542	0.6300	0.099*
H22C	−0.4460	0.6470	0.6431	0.099*
O3	0.1692 (3)	0.3887 (2)	0.80660 (12)	0.0721 (7)
N3	0.1779 (4)	0.4295 (2)	0.87181 (16)	0.0564 (7)
O2	0.1035 (4)	0.3954 (2)	0.92659 (14)	0.0901 (9)
O1	0.2677 (3)	0.5050 (2)	0.88344 (16)	0.0796 (8)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
N1	0.0432 (13)	0.0476 (14)	0.0338 (12)	0.0053 (12)	0.0035 (11)	−0.0016 (10)
N2	0.0445 (15)	0.0562 (15)	0.0306 (12)	0.0085 (12)	0.0007 (11)	0.0029 (11)
C16	0.0384 (15)	0.0424 (15)	0.0419 (16)	0.0045 (13)	−0.0010 (15)	−0.0022 (14)
C8	0.0360 (15)	0.0432 (16)	0.0413 (16)	0.0010 (14)	−0.0024 (13)	0.0032 (13)
C19	0.0426 (16)	0.0425 (15)	0.0541 (18)	0.0026 (14)	−0.0014 (16)	−0.0071 (15)
C7	0.0331 (14)	0.0398 (15)	0.0376 (15)	0.0001 (13)	0.0015 (12)	−0.0009 (12)
C15	0.0402 (15)	0.0446 (16)	0.0374 (16)	0.0030 (13)	0.0008 (14)	0.0038 (14)
C6	0.0367 (15)	0.0470 (16)	0.0330 (14)	−0.0035 (14)	0.0009 (12)	0.0024 (12)
C1	0.0410 (16)	0.0520 (17)	0.0456 (17)	0.0058 (15)	−0.0002 (14)	0.0000 (15)
C21	0.0508 (18)	0.0513 (17)	0.0426 (17)	0.0061 (16)	0.0029 (15)	0.0078 (14)
C10	0.0453 (17)	0.0503 (17)	0.0522 (18)	0.0002 (15)	−0.0004 (14)	−0.0071 (15)
C17	0.0475 (17)	0.0576 (18)	0.0396 (17)	0.0040 (15)	0.0006 (15)	0.0048 (14)
C9	0.0432 (16)	0.0400 (15)	0.0387 (15)	0.0063 (14)	0.0025 (13)	0.0037 (13)
C12	0.057 (2)	0.068 (2)	0.0424 (18)	0.0168 (17)	0.0100 (17)	−0.0034 (17)
C20	0.057 (2)	0.0465 (17)	0.0460 (18)	0.0091 (16)	−0.0023 (15)	0.0046 (15)
C14	0.0473 (19)	0.075 (2)	0.0425 (18)	0.0035 (17)	−0.0005 (15)	−0.0022 (16)
C13	0.062 (2)	0.082 (2)	0.0386 (18)	0.0152 (19)	−0.0057 (16)	−0.0114 (17)
C4	0.070 (2)	0.068 (2)	0.0398 (17)	−0.014 (2)	−0.0128 (17)	0.0063 (16)
C11	0.0453 (19)	0.062 (2)	0.066 (2)	−0.0002 (17)	0.0112 (17)	−0.0041 (18)
C18	0.0433 (16)	0.0556 (18)	0.0435 (17)	−0.0022 (15)	0.0109 (14)	−0.0055 (15)
C5	0.0542 (19)	0.0508 (18)	0.0396 (17)	−0.0052 (17)	−0.0005 (15)	0.0020 (14)
C2	0.0446 (19)	0.067 (2)	0.063 (2)	0.0116 (17)	−0.0022 (17)	0.0084 (18)
C3	0.0506 (19)	0.083 (2)	0.057 (2)	−0.0026 (19)	−0.0162 (17)	0.023 (2)
C22	0.0498 (19)	0.064 (2)	0.085 (2)	0.0097 (16)	−0.002 (2)	−0.001 (2)
O3	0.0800 (17)	0.1055 (19)	0.0309 (12)	0.0005 (15)	−0.0044 (12)	−0.0037 (13)
N3	0.0602 (18)	0.0612 (18)	0.0477 (17)	0.0134 (15)	−0.0019 (15)	0.0046 (14)
O2	0.142 (3)	0.0865 (19)	0.0423 (13)	−0.011 (2)	0.0201 (16)	0.0009 (14)
O1	0.0746 (17)	0.0722 (16)	0.092 (2)	−0.0013 (15)	0.0004 (17)	−0.0109 (15)

Geometric parameters (Å, º) top

N1—C15	1.333 (3)	C17—H17	0.9300
N1—C7	1.381 (3)	C9—C14	1.383 (4)
N1—H1A	0.8600	C12—C13	1.364 (5)
N2—C15	1.324 (3)	C12—C11	1.369 (4)
N2—C8	1.379 (4)	C12—H12	0.9300
N2—H2A	0.8600	C20—H20	0.9300
C16—C21	1.385 (4)	C14—C13	1.375 (4)
C16—C17	1.386 (4)	C14—H14	0.9300
C16—C15	1.454 (4)	C13—H13	0.9300
C8—C7	1.367 (4)	C4—C3	1.376 (5)
C8—C9	1.469 (4)	C4—C5	1.382 (4)
C19—C18	1.377 (4)	C4—H4	0.9300
C19—C20	1.379 (4)	C11—H11	0.9300
C19—C22	1.499 (4)	C18—H18	0.9300
C7—C6	1.467 (4)	C5—H5	0.9300
C6—C5	1.384 (4)	C2—C3	1.378 (5)
C6—C1	1.387 (4)	C2—H2	0.9300
C1—C2	1.372 (4)	C3—H3	0.9300
C1—H1	0.9300	C22—H22A	0.9600
C21—C20	1.370 (4)	C22—H22B	0.9600
C21—H21	0.9300	C22—H22C	0.9600
C10—C9	1.372 (4)	O3—N3	1.236 (3)
C10—C11	1.377 (4)	N3—O2	1.212 (3)
C10—H10	0.9300	N3—O1	1.254 (4)
C17—C18	1.380 (4)

C15—N1—C7	110.5 (2)	C13—C12—C11	119.8 (3)
C15—N1—H1A	124.7	C13—C12—H12	120.1
C7—N1—H1A	124.7	C11—C12—H12	120.1
C15—N2—C8	111.3 (2)	C21—C20—C19	121.8 (3)
C15—N2—H2A	124.4	C21—C20—H20	119.1
C8—N2—H2A	124.4	C19—C20—H20	119.1
C21—C16—C17	118.5 (3)	C13—C14—C9	120.8 (3)
C21—C16—C15	120.4 (3)	C13—C14—H14	119.6
C17—C16—C15	121.1 (3)	C9—C14—H14	119.6
C7—C8—N2	105.6 (2)	C12—C13—C14	120.0 (3)
C7—C8—C9	134.2 (3)	C12—C13—H13	120.0
N2—C8—C9	120.2 (2)	C14—C13—H13	120.0
C18—C19—C20	117.5 (3)	C3—C4—C5	119.9 (3)
C18—C19—C22	121.1 (3)	C3—C4—H4	120.1
C20—C19—C22	121.4 (3)	C5—C4—H4	120.1
C8—C7—N1	106.3 (2)	C12—C11—C10	120.4 (3)
C8—C7—C6	132.6 (3)	C12—C11—H11	119.8
N1—C7—C6	121.2 (2)	C10—C11—H11	119.8
N2—C15—N1	106.3 (2)	C19—C18—C17	121.8 (3)
N2—C15—C16	126.8 (3)	C19—C18—H18	119.1
N1—C15—C16	126.9 (3)	C17—C18—H18	119.1
C5—C6—C1	119.1 (3)	C4—C5—C6	120.3 (3)
C5—C6—C7	120.0 (3)	C4—C5—H5	119.8
C1—C6—C7	120.9 (3)	C6—C5—H5	119.8
C2—C1—C6	120.4 (3)	C1—C2—C3	120.1 (3)
C2—C1—H1	119.8	C1—C2—H2	119.9
C6—C1—H1	119.8	C3—C2—H2	119.9
C20—C21—C16	120.4 (3)	C4—C3—C2	120.1 (3)
C20—C21—H21	119.8	C4—C3—H3	120.0
C16—C21—H21	119.8	C2—C3—H3	120.0
C9—C10—C11	120.3 (3)	C19—C22—H22A	109.5
C9—C10—H10	119.8	C19—C22—H22B	109.5
C11—C10—H10	119.8	H22A—C22—H22B	109.5
C18—C17—C16	120.0 (3)	C19—C22—H22C	109.5
C18—C17—H17	120.0	H22A—C22—H22C	109.5
C16—C17—H17	120.0	H22B—C22—H22C	109.5
C10—C9—C14	118.7 (3)	O2—N3—O3	120.7 (3)
C10—C9—C8	121.3 (3)	O2—N3—O1	118.4 (3)
C14—C9—C8	120.0 (3)	O3—N3—O1	120.9 (3)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N2—H2A···O1ⁱ	0.86	2.05	2.905 (4)	176
N2—H2A···O2ⁱ	0.86	2.39	2.922 (3)	121
N1—H1A···O3	0.86	1.96	2.720 (3)	147

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

Experimental details

Crystal data
Chemical formula	C₂₂H₁₉N₂⁺·NO₃⁻
M_r	373.40
Crystal system, space group	Orthorhombic, P2₁2₁2₁
Temperature (K)	298
a, b, c (Å)	8.442 (2), 12.970 (3), 17.098 (3)
V (Å³)	1872.3 (6)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.09
Crystal size (mm)	0.40 × 0.35 × 0.25

Data collection
Diffractometer	Rigaku Mercury2 diffractometer
Absorption correction	Multi-scan (CrystalClear; Rigaku, 2005)
T_min, T_max	0.955, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	19745, 4278, 2714
R_int	0.089
(sin θ/λ)_max (Å⁻¹)	0.649

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.059, 0.143, 1.03
No. of reflections	4278
No. of parameters	254
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.26, −0.20

Computer programs: CrystalClear (Rigaku, 2005), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL/PC (Sheldrick, 2008).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N2—H2A···O1ⁱ	0.86	2.05	2.905 (4)	175.6
N2—H2A···O2ⁱ	0.86	2.39	2.922 (3)	120.9
N1—H1A···O3	0.86	1.96	2.720 (3)	147.3

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

Acknowledgements

This work was supported by a start-up grant from Southeast University to Professor Ren-Gen Xiong.

References

Dai, W. & Fu, D.-W. (2008). Acta Cryst. E64, o971. Web of Science CSD CrossRef IUCr Journals Google Scholar
Fu, D.-W. & Xiong, R.-G. (2008). Dalton Trans. pp. 3946–3948. Web of Science CSD CrossRef Google Scholar
Huang, X.-F., Fu, D.-W. & Xiong, R.-G. (2008). Cryst. Growth. Des. 8, 1795-1797. Web of Science CSD CrossRef CAS Google Scholar
Rigaku (2005). CrystalClear. Rigaku Corporation, Tokyo, Japan. Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar