Crystal Growth & Design
Article
Table 1. Crystallographic Data and Structural Refinement Parameters
[C(NH2)3]HC2O4·H2O
[C(NH2)3]PO2H2
[C(NH2)3]2(PF6)·F
[C(NH2)3]CF3COO
[C(NH2)3]Br
formula weight
crystal system
space group
a (Å)
b (Å)
c (Å)
167.13
monoclinic
P21/c
6.7009(8)
10.5439(13)
10.2231(12)
β = 103.783(6)
701.50(15)
4
125.07
orthorhombic
Pnma
9.5739(5)
8.5447(4)
7.4084(4)
284.15
monoclinic
C2/c
18.5498(13)
8.4623(6)
15.0860(10)
β = 112.023(4)
2195.3(3)
8
173.11
orthorhombic
Pbcn
10.5705(13)
10.2525(13)
13.0173(15)
140.00
orthorhombic
Pnma
8.6650(13)
6.8074(9)
8.3304(11)
β (deg)
V (Å3)
606.05(5)
4
1.371
1410.7(3)
8
1.630
491.38(12)
4
1.892
Z
dcal (g/cm3)
1.582
1.320
1.707
3.130
μ (mm−1
)
3.367
0.180
8.201
GOOF
R1 (I > 2σ(I)])
wR2 (I > 2σ(I)])
R1 (all data)
1.068
0.0602
0.1277
0.1140
0.1494
0.264/−0.231
1.076
1.046
0.0513
0.1269
0.0888
0.1437
0.532/−0.327
1.056
1.037
a
0.0513
0.1197
0.0972
0.1413
0.255/−0.252
0.0655
0.1783
0.1003
0.2051
0.541/−0.404
0.0216
0.0390
0.0372
0.0430
0.319/−0.252
a
a
a
wR2 (all data)
largest diff. peak and hole e·Å−3
R1 = ∑∥Fo| − |Fc∥/∑|Fo| and wR2 = [∑w(Fo − Fc2)2/∑wFo ]1/2 for Fo > 2σ(Fo ).
a
2
4
2
2
2.4. Powder X-ray Diffraction and Thermal Behavior
Analysis. The powder XRD measurements of [C(NH2)3]HC2O4·
H2O, [C(NH2)3]PO2H2, [C(NH2)3]CF3COO, [C(NH2)3]Br, and
[C(NH2)3]2(PF6)·F were carried out on a Bruker D2 PHASER
diffractometer (λ = 1.5418 Å and 2θ = 10−70°). As shown in Figure
S1 in the SI, the observed XRD patterns of the five compounds are in
good agreement with the theoretical ones. Thermal gravimetric (TG)
and differential scanning calorimetry (DSC) of the five compounds
were investigated using a NETZSCH STA 449C simultaneous
thermal analyzer. All the samples were placed in different platinum
crucibles and heated at a rate of 5 °C/min from 40 to 450 °C with the
[C(NH2)3]2(PF6)·F is obtained for the first time; the other
four compounds are known ones without evaluating optical
properties in detail.41−44 As expected, all of them exhibit large
birefringence and short UV cutoff edges, which indicates that
they can be applied as birefringent crystals. More importantly,
[C(NH2)3]HC2O4·H2O possesses a giant birefringence of Δn
= 0.371@532 nm, which is even larger than that of most of the
commercialized UV birefringent crystals. Our findings
demonstrate that the guanidine-based compound is a novel
source of short-wavelength birefringent crystals.
2.5. Vibrational Spectroscopy and Optical Properties. The
infrared spectroscopy was carried out on a Shimadzu IRAffinity-1
spectrometer. The samples of [C(NH2)3]HC2O4·H2O, [C(NH2)3]-
PO2H2, [C(NH2)3]2(PF6)·F, [C(NH2)3]CF3COO, and [C(NH2)3]Br
were mixed, ground with dried KBr, and then collected in the range
from 450 to 4000 cm−1 with a resolution of 2 cm−1. The UV−vis−
NIR diffuse-reflectance data for the five polycrystalline powders were
collected by a Shimadzu Solid Spec-3700DUV Spectrophotometer
with the measurement range extended from 190 to 1000 nm.
2.6. Computational Methods. The electronic structures of the
five compounds were studied by density functional theory based on ab
initio calculations.48 The exchange-correlation potential was calculated
using the Perdew−Burke−Ernzerhof functional within the generalized
gradient approximation. The following orbital electrons were treated
as valence electrons:49 H: 1s2, C: 2s22p2, N: 2s22p3, F: 2s22p5, O:
2s22p4, P: 3s23p3, Br: 4s24p5. To achieve energy convergence, a plane-
wave basis set energy cutoff was 700, 850, 750, 850, and 700 eV
within the normal-conserving pseudo-potential for [C(NH2)3]-
HC2O4·H2O, [C(NH2)3]PO2H2, [C(NH2)3]2(PF6)·F, [C(NH2)3]-
CF3COO, and [C(NH2)3]Br. The Monkhorst−Pack scheme were set
as 4 × 2 × 3, 3 × 3 × 3, 4 × 4 × 2, 4 × 4 × 3, and 3 × 4 × 3 in the
Brillouin zone of the primitive cells.50 These basic microscopic
properties, including anisotropy polarizability and energy band gap,
were calculated by buliding the geometric models of [C(NH2)3] and
[C2O4] units from related title compounds by Gaussian software,
which is based on the B3LYP (Becke, three-parameter, Lee−Yang−
Parr) cross-correlation functional with the Lee−Yang−Parr cross-
correlation.51
2. EXPERIMENTAL SECTION
2.1. Experimental Reagents. All the chemical reagents covering
guanidine carbonate (99%), H2C2O4·2H2O (99.5%), H3PO2 (50 wt %
in H2O), HPF6 (∼60 wt % in H2O), CF3COOH (98%), and HBr
(AR, 40%) were purchased and used directly to synthesize the target
title compounds.
2.2. Synthesis of Single Crystal. All of the five compounds were
synthesized by a simple aqueous solution volatilization method. The
chemical reagents were dissolved in the deionized water (20 mL) with
the following stoichiometry: guanidine carbonate:H2C2O4·2H2O/
H3PO2/CF3COOH/HBr/HPF6 = 1:1 for [C(NH2)3]HC2O4·H2O,
[C(NH2)3]PO2H2, [C(NH2)3]CF3COO, and [C(NH2)3]Br, as well
as guanidine carbonate:HPF6 = 2:1 for [C(NH2)3]2(PF6)·F,
respectively. These solutions were heated to 50 °C and kept for
evaporation to dryness at room temperature. Finally, all the five target
single crystals were successfully obtained.
2.3. Single-Crystal Structure Diffraction. Five colorless single
crystals with different sizes (see detail in Table 1) were selected for
single-crystal X-ray diffraction. All diffraction data were collected on a
Bruker D8 Venture diffractometer equipped with Cu Kα (λ = 1.5406
Å) radiation at 298 K/153 K. The intensity, reduction, and cell
refinement were carried out on a Bruker SAINT.45 All the structures
were solved by a direct method and refined through the full-matrix
least-squares fitting on F2 with SHELX and OLEX2 software.46 These
structures were verified by virtue of the ADDSYM algorithm from
PLATON.47 Crystal data and details of the crystal parameters are
listed in Table 1. The atomic coordinates equivalent isotropic
displacement parameters and selected bond lengths are summarized
solved crystal structures of [C(NH2)3]HC2O4·H2O, [C(NH2)3]-
CF3COO, [C(NH2)3]PO2H2, and [C(NH2)3]Br are well consistent
with the previous studies.41−44
3. RESULTS AND DISCUSSION
3.1. Crystal Structure Description. [C(NH2)3]HC2O4·H2O.
[C(NH2)3]HC2O4·H2O crystallizes in the monoclinic crystal
system with space group P21/c,41 and it displays 2D
1870
Cryst. Growth Des. 2021, 21, 1869−1877