L-Leucinium perchlorate

Crystals of L-leucinium perchlorate, C₆H₁₄NO₂⁺·ClO₄⁻, are built up from protonated L-leucinium cations and perchlorate anions. L-Leucinium cations related by a twofold screw axis are inter­connected by N—H...O hydrogen bonds into zigzag chains parallel to [010]. The O atoms of the perchlorate anions act as acceptors of hydrogen bonds that link the L-leucinium chains into separated but inter­acting two-dimensional layers parallel to (001). Since the title compound crystallizes in a non-centrosymmetric space group, it can be useful as a material for non-linear optics. The efficiency of second harmonic generation is about twice that of K₂[HPO₄].

S1. Comment

The present study is a continuation of our investigations characterizing the hydrogen-bonding system in the solid state of materials that generate SHG (second harmonic generation) (Janczak & Perpétuo, 2002; Marchewka et al., 2003; Perpétuo & Janczak, 2006). In order to expand understanding of the solid-state physical-organic chemistry of compounds for nonlinear optics, we investigate here the solid-state structure of L-leucinium perchlorate, (I). Additionally, the geometries of the oppositely charged units of the crystal, i.e. the protonated L-leucinium cation and the perchlorate anion, are compared with ab initio optimized parameters calculated at the B3LYP/6–31 G(d) level (Frisch et al., 1998) and the results are shown in Fig. 1.

The asymmetric unit of (I) consists of a protonated L-leucinium cation and a perchlorate anion (Fig. 2). The protonated L-leucinium cation adopts an extended conformation, where the methyl groups are at a maximum distance from the polar groups, making the C—C bonds slightly longer than a typical single C—C bond [1.515 (5) Å; Allen et al., 1987]. The steric effect and interaction between the two methyl groups and both polar NH₃⁺ and COOH groups makes the C2—C3—C4 angle larger than the remaining C—C—C angles within the L-leucinium skeleton. The protonated amine group interacts with the polar COOH group, making the C1—C2—N1 angle slightly smaller than the expected value of 109.5°. The correlations between the values of the C—C bond lengths and C—C—C angles within the skeleton are more pronounced in the ab-inito optimized geometry of the L-leucinium cation. The extended conformation of the L-leucinium cation is described well by the orientation of the polar NH₃⁺ and COOH groups in relation to the carbon skeleton.

The orientation of the NH₃⁺ group is almost coplanar with the C2—C3—C4—C6 chain [the N1—C2—C3—C4 and C2—C3—C4—C6 torsion angles are -170.6 (2) and -177.5 (2)°, respectively], while the COOH group and the other methyl group (C5) deviate significantly from this skeleton plane [the C1—C2—C3—C4 and C5—C4—C3—C2 torsion angles are 70.0 (2) and 60.9 (2)°, respectively]. A similar extended conformation of the skeleton is observed in the crystal structure of L-leucine (Coll et al., 1986; Görbitz & Dalhus, 1996) and in the almost all known crystals of L-leucine with organic or inorganic acids (Allen, 2002), except for L-leucinium oxalate (Rajagopal et al., 2003) and L-leucine L-leucinium picrate (Anitha et al., 2005), in each of which the C atom of the carboxyl group is almost coplanar with the C2—C3—C4—C6 skeleton.

The perchlorate anion shows noticeable distortion from the tetrahedral geometry predicted by the ab-initio optimization (Cl—O = 1.499 Å and O—Cl—O = 109.49°); firstly the observed Cl—O bonds are different [1.389 (2)–1.413 (2) Å] and secondly the O—Cl—O angles are not equivalent, ranging from 106.7(2) to 115.7 (2)°. The distortion of ClO₄^- from T_d symmetry is most likely due to the different interactions of the O atoms with the L-leucinium cations that act as an acceptor in the hydrogen bonds.

In the crystal structure the L-leucinium cations related by a twofold screw axis interact via N—H···O hydrogen bonds, forming infinite zigzag chains parallel to the b axis. The O atoms of the perchlorate anion as acceptors of the hydrogen bond interconnect the translationally equivalent chains of L-leucinium cations into two-dimensional layers perpendicular to the c axis (Fig. 3). Within the layers, the N—H···O interaction between the L-leucinium cations and the interionic interactions between the L-leucinium(+) and ClO₄^- ions (N—H···O and O—H···O) are stronger than that between the layers. There are no direction-specific interactions between adjacent sheets, accounting for the marked cleavage properties of the crystals, but the isobutyl groups of L-leucinium units in adjacent sheets are interdigitated (Fig. 2).

Two H atoms of the protonated amine group are each involved in two N—H···O hydrogen bonds, while the third forms one N—H···O bond (see Table 2). Besides these N—H···O hydrogen bonds, the L-leucinium cation is involved also, via the OH group, in an O—H···O hydrogen bond with the ClO₄^- anion, which acts as an acceptor in five hydrogen bonds. Atoms O1, O2 and O3 are each acceptors in one hydrogen bond, while atom O4 is involved in two such interactions (Table 2).

The SHG experiment was carried out using the Kutz–Perry powder technique (Kurtz & Perry, 1968). Samples of L-leucinium perchlorate and K₂[HPO₄] (KDP) were irradiated at 1064 nm by an Nd:YAG laser and the second harmonic beam power diffused by the samples at 532 nm was measured as a function of the fundamental beam power. SHG efficiency for L-leucinium perchlorate is about two times larger than for KDP [d_eff ≈ 2d_eff(KDP)].

Related literature

For related literature, see: Allen (2002); Allen et al. (1987); Anitha et al. (2005); Coll et al. (1986); Frisch et al. (1998); Görbitz & Dalhus (1996); Janczak & Perpétuo (2002); Kurtz & Perry (1968); Marchewka et al. (2003); Perpétuo & Janczak (2006); Rajagopal et al. (2003).

S2. Experimental

L-Leucine was dissolved in 10% aqueous perchloric acid; after several days, colourless single crystals had formed, which proved to be suitable for single-crystal X-ray diffraction analysis.

S3. Refinement

All H atoms were treated as riding atoms with C—H distances of 0.96–0.98 Å, N—H distances of 0.89 Å and O—H distances of 0.82 Å, with U_iso(H) set equal to kU_eq(C,N,O), where k = 1.5 for methyl, ammonium and O—H, and k = 1.2 for all other H atoms. In the absence of significant resonance scattering, the Friedel-equivalent reflections were merged; the absolute structure was assigned from the known configuration of the leucine component.

Computing details

Data collection: KM-4 CCD Software (Kuma, 2002 or 2001); cell refinement: KM-4 CCD Software; data reduction: KM-4 CCD Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Sheldrick, 1990); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. Results of the optimized molecular orbital calculations for the L-leucinium cation.
	Fig. 2. A view of the molecular structure of (I), showing displacement ellipsoids at the 50% probability level and H atoms as spheres of arbitrary radii.
	Fig. 3. The crystal packing, viewed approximately along [100], showing three (001) sheets and their interdigitated isobutyl groups. For the sake of clarity, H atoms bonded to C atoms have been omitted.

L-Leucinium perchlorate

Crystal data

C6H14NO2^+·ClO4^−	F(000) = 244
Mr = 231.63	Dx = 1.455 Mg m^−3 Dm = 1.45 Mg m^−3 Dm measured by flotation in what???
Monoclinic, P21	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2yb	Cell parameters from 1972 reflections
a = 5.640 (1) Å	θ = 3.0–28.6°
b = 8.768 (2) Å	µ = 0.37 mm^−1
c = 10.704 (2) Å	T = 295 K
β = 92.49 (3)°	Parallelepiped, colourless
V = 528.83 (18) Å^3	0.28 × 0.22 × 0.18 mm
Z = 2

Data collection

Kuma KM-4 with CCD area-detector diffractometer	2599 independent reflections
Radiation source: fine-focus sealed tube	1972 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.020
Detector resolution: 1024x1024 with blocks 2x2 pixels mm^-1	θmax = 28.6°, θmin = 3.0°
ω scans	h = −7→5
Absorption correction: analytical (face-indexed; SHELXTL; Sheldrick, 1990)	k = −11→11
Tmin = 0.915, Tmax = 0.941	l = −13→14
6261 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.042	H-atom parameters constrained
wR(F^2) = 0.107	w = 1/[σ^2(Fo^2) + (0.0658P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.00	(Δ/σ)max < 0.001
2599 reflections	Δρmax = 0.30 e Å^−3
131 parameters	Δρmin = −0.30 e Å^−3
1 restraint	Absolute structure: Flack (1983), 1162 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.13 (8)

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
Cl1	1.00984 (9)	0.39147 (7)	0.36571 (5)	0.05757 (17)
O1	0.8567 (4)	0.2661 (3)	0.3427 (2)	0.0617 (7)
O2	1.0241 (6)	0.4786 (3)	0.2554 (2)	0.0658 (10)
O3	0.9507 (6)	0.4867 (4)	0.4633 (3)	0.0663 (13)
O4	1.2365 (6)	0.3363 (4)	0.3952 (4)	0.0682 (17)
O5	0.5844 (4)	0.2423 (2)	0.6213 (2)	0.0581 (6)
O6	0.9090 (4)	0.2888 (3)	0.7401 (2)	0.0725 (7)
H6	0.9662	0.2153	0.7049	0.109*
C1	0.7108 (6)	0.3290 (3)	0.6808 (3)	0.0436 (7)
C2	0.6537 (5)	0.4939 (3)	0.6951 (2)	0.0355 (6)
H2	0.8012	0.5531	0.6964	0.053*
N1	0.5004 (4)	0.5401 (2)	0.5844 (2)	0.0438 (5)
H11	0.5874	0.5441	0.5171	0.066*
H12	0.4379	0.6314	0.5981	0.066*
H13	0.3845	0.4721	0.5721	0.066*
C3	0.5240 (4)	0.5252 (3)	0.8158 (2)	0.0413 (5)
H31	0.4619	0.6283	0.8117	0.062*
H32	0.3895	0.4565	0.8182	0.062*
C4	0.6703 (5)	0.5081 (4)	0.9378 (2)	0.0463 (6)
H4	0.7319	0.4037	0.9431	0.073*
C5	0.8792 (6)	0.6186 (6)	0.9441 (3)	0.0684 (13)
H51	0.9859	0.5942	0.8796	0.125*
H52	0.9614	0.6103	1.0243	0.125*
H53	0.8219	0.7210	0.9326	0.125*
C6	0.5135 (7)	0.5360 (6)	1.0489 (3)	0.0695 (11)
H61	0.6056	0.5216	1.1255	0.119*
H62	0.3832	0.4655	1.0454	0.119*
H63	0.4536	0.6385	1.0454	0.119*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cl1	0.0596 (3)	0.0549 (3)	0.0579 (3)	−0.0046 (3)	−0.0020 (2)	−0.0003 (3)
O1	0.0625 (16)	0.0548 (14)	0.0680 (16)	−0.0408 (12)	0.0043 (12)	−0.0029 (12)
O2	0.081 (3)	0.0599 (17)	0.0546 (15)	−0.0509 (18)	−0.0163 (15)	0.0170 (12)
O3	0.087 (3)	0.086 (2)	0.092 (2)	−0.038 (2)	0.030 (2)	−0.038 (2)
O4	0.087 (2)	0.076 (2)	0.094 (5)	0.0327 (16)	−0.037 (3)	−0.027 (2)
O5	0.0778 (15)	0.0331 (10)	0.0613 (13)	0.0049 (11)	−0.0208 (11)	−0.0076 (9)
O6	0.0821 (17)	0.0517 (13)	0.0805 (16)	0.0356 (12)	−0.0317 (13)	−0.0216 (11)
C1	0.0577 (18)	0.0343 (13)	0.0376 (14)	0.0119 (12)	−0.0109 (13)	−0.0049 (11)
C2	0.0411 (14)	0.0268 (12)	0.0378 (12)	0.0016 (10)	−0.0065 (10)	0.0007 (10)
N1	0.0547 (14)	0.0343 (11)	0.0414 (12)	0.0117 (10)	−0.0093 (9)	−0.0006 (9)
C3	0.0357 (13)	0.0496 (14)	0.0381 (12)	0.0047 (11)	−0.0033 (10)	−0.0045 (11)
C4	0.0470 (15)	0.0488 (16)	0.0450 (12)	0.0051 (14)	0.0004 (11)	0.0042 (12)
C5	0.062 (2)	0.092 (4)	0.0501 (18)	−0.026 (2)	−0.0086 (16)	−0.010 (2)
C6	0.069 (2)	0.095 (3)	0.0454 (17)	0.012 (2)	0.0092 (15)	0.003 (2)

Geometric parameters (Å, º)

Cl1—O3	1.389 (2)	N1—H13	0.8900
Cl1—O4	1.390 (3)	C3—C4	1.521 (4)
Cl1—O2	1.412 (3)	C3—H31	0.9700
Cl1—O1	1.413 (2)	C3—H32	0.9700
O5—C1	1.205 (3)	C4—C5	1.525 (5)
O6—C1	1.310 (3)	C4—C6	1.532 (4)
O6—H6	0.8200	C4—H4	0.9800
C1—C2	1.491 (4)	C5—H51	0.9600
C2—N1	1.492 (3)	C5—H52	0.9600
C2—C3	1.537 (4)	C5—H53	0.9600
C2—H2	0.9800	C6—H61	0.9600
N1—H11	0.8900	C6—H62	0.9600
N1—H12	0.8900	C6—H63	0.9600
O3—Cl1—O4	106.7 (2)	C4—C3—H31	108.2
O3—Cl1—O2	109.1 (2)	C2—C3—H31	108.2
O4—Cl1—O2	107.0 (3)	C4—C3—H32	108.2
O3—Cl1—O1	115.71 (16)	C2—C3—H32	108.2
O4—Cl1—O1	108.55 (19)	H31—C3—H32	107.4
O2—Cl1—O1	109.40 (15)	C3—C4—C5	111.4 (2)
C1—O6—H6	109.5	C3—C4—C6	109.9 (2)
O5—C1—O6	124.0 (3)	C5—C4—C6	109.5 (3)
O5—C1—C2	122.7 (3)	C3—C4—H4	108.6
O6—C1—C2	113.2 (3)	C5—C4—H4	108.6
C1—C2—N1	107.5 (2)	C6—C4—H4	108.6
C1—C2—C3	111.9 (3)	C4—C5—H51	109.5
N1—C2—C3	109.8 (2)	C4—C5—H52	109.5
C1—C2—H2	109.2	H51—C5—H52	109.5
N1—C2—H2	109.2	C4—C5—H53	109.5
C3—C2—H2	109.2	H51—C5—H53	109.5
C2—N1—H11	109.5	H52—C5—H53	109.5
C2—N1—H12	109.5	C4—C6—H61	109.5
H11—N1—H12	109.5	C4—C6—H62	109.5
C2—N1—H13	109.5	H61—C6—H62	109.5
H11—N1—H13	109.5	C4—C6—H63	109.5
H12—N1—H13	109.5	H61—C6—H63	109.5
C4—C3—C2	116.3 (2)	H62—C6—H63	109.5

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H11···O3	0.89	2.21	2.939 (4)	139
O6—H6···O2^i	0.82	2.12	2.746 (3)	133
N1—H12···O1^ii	0.89	2.16	2.954 (3)	149
N1—H12···O4^iii	0.89	2.57	2.995 (4)	110
N1—H13···O4^iv	0.89	2.36	3.041 (4)	134
N1—H11···O5^ii	0.89	2.45	2.852 (3)	108

Symmetry codes: (i) −x+2, y−1/2, −z+1; (ii) −x+1, y+1/2, −z+1; (iii) −x+2, y+1/2, −z+1; (iv) x−1, y, z.