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Trioctylphosphine oxide (TOPO), C24H51OP, was recrystallized from ambient evaporation in acetone. TOPO single crystals form with a monoclinic P21/c structure. Fourier transform IR (FT-IR) spectroscopy captures the characteristic stretching modes from the seven methyl­ene groups, the phosphoryl P=O bond, and the phosphor­yl-carbon bond.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S2053229615003009/fn3187sup1.cif
Contains datablock I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S2053229615003009/fn3187Isup2.hkl
Contains datablock I

cml

Chemical Markup Language (CML) file https://doi.org/10.1107/S2053229615003009/fn3187Isup3.cml
Supplementary material

CCDC reference: 1048929

Introduction top

Trioctylphosphine oxide (TOPO) is a widely used chemical compound in nanocrystal synthesis, for the removal of heavy metals, and removal of toxins in waste water (Praveen & Loh, 2013). TOPO is often used as a ligand stabilizer for colloids in traditional thermal decomposition synthetic techniques (Manna et al., 2000; Murray et al., 1993; Rockenberger et al., 1998; Talapin et al., 2001). The white crystalline material is also used as a phospho­rus source, as the compound decomposes in high-temperature (> 573 K) reactions for colloidal synthesis (Zhang et al., 2011) and has been characterized by nuclear magnetic resonance (Hens et al., 2005; Hilliard et al., 2012; Kriz et al., 2009; Liu et al., 2007). In this report, we provide additional spectroscopic data for TOPO beyond the single-crystal diffraction results.

Experimental top

Synthesis and crystallization top

Trioctylphosphine oxide (90%) was purchased from Sigma–Aldrich and acetone (99.5% reagent grade) was purchased from Fisher Scientific. TOPO (0.5 g) was dissolved in acetone (5 ml) in a 25 ml scintillation vial, which was left uncapped for ambient evaporation at 293 K for 12 h. Single crystals were collected and loaded onto a Kapton capillary tube for measurement.

Refinement top

Crystal data, data collection and structure refinement details are summarised in Table 1. H atoms were refined using a riding model, in which standard C—H bond distances were applied, and the H-atom positions were adjusted during refinement. Isotropic displacement parameters of methyl­ene H atoms were assigned as 33% larger than the attached C atom, while methyl H atoms had displacement parameters 50% larger. In addition, the methyl torsion angle (for rotation about the C—Me bond) was optimized by the refinement program.

Trioctylphosphine oxide (0.05 g) was mixed with potassium bromide and pressed to form a pellet for FT–IR analysis, which was carried out using a Nicolet spectrometer and averaged for 16 scans in a nitro­gen atmosphere to reduce molecular vibrations from ambient environment. Raman spectroscopy was performed using a B&W Tek iRaman spectrometer with a 532 nm excitation source. Modeling of the X-ray scattering patterns were done using an in-house code (Gordon et al., 2015) calculating the Debye Equation (Equation 1). In computing the X-ray scattering intensities for the 2 to 8 nm crystallite sizes of trioctylphosphine oxide, the parameters fi and fj are the atomic scattering factors tabulated from Cromer–Mann coefficients, q is the wave vector in Å-1, rij is the pairwise distance in Å (Brown et al., 2004; Debye, 1915; Hovestreydt, 1983). The bulk crystal simulation was performed using CrystalDiffract (CrystalMaker, 2015).

<fi>I = Σi,j fifj (sin(qrij))/(qrij)</fi> (1)

Results and discussion top

Slow evaporation resulted in a mixture of single crystals, of which the highest-quality single crystals were collected. The molecular structure of TOPO is shown in Fig. 1 (Johnson, 1976). The unit cell is drawn more compactly in Fig. 2, where the phospho­nyl group within each molecular unit is oriented such that the polar functional groups are 180° from each other. Each O atom is 3.94 Å from the P atom on the nearest neighboring molecular unit. The two-dimensional projection of the unit cell onto the (010) plane depicts the alternating directions of the dipole from the PO bonds. The flexible hydro­carbon chains are aligned parallel to neighboring octyl chains, which allows for compact packing of the molecular units.

In processes that require careful removal of excess TOPO from solution, such as post-synthesis nanocrystal purification, total X-ray scattering provides high-resolution and rapid acquisition of data to track TOPO impurities that will crystallize with varying grain sizes. The X-ray scattering for the 77-atom unit cell was calculated as a function of crystallite size for comparison with the total X-ray scattering data obtained with a synchrotron light source (λ = 0.2114 Å).

The simulations in Fig. 3(a) track the peak width broadening as the crystal size decreased from an infinite crystal to a few unit cells. The decrease in the inverse of full-width half-maximum of the Gaussian fits is linearly correlated with the crystal size (Fig. 3b) and matches well with the collected scattering data. The slope of the fit for crystal diameter as a function of the inverse of full-width half maximum was calculated to be 1.101±0.323 for the dominant (114) set of planes, which corresponds with the κλ/cos(θ) in which κ is the shape factor of value 0.9, λ is the X-ray wavelength of value 0.2114 Å, and θ is the peak center of value 1.40 Å-1. Spectroscopy from FT–IR shows the –CH2 stretching modes in the three alkyl chains at 2850 and 2919 cm-1. The characteristic PO and P—C stretching modes were observed at 1146 and 1465 cm-1, respectively.

The complementary Raman spectroscopic data, which has previously been unreported, confirms the phospho­nyl stretching mode in a tri­alkyl­phospho­nyl environment results in a much weaker peak at 1145 cm-1 than the dominant methyl­ene stretching modes at 2847 and 2882 cm-1 (Fig. 5).

Computing details top

Data collection: APEX2 (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. Refined ORTEP schematic for monoclinic trioctylphosphine oxide with 50% probability displacement ellipsoids.
[Figure 2] Fig. 2. A two-dimensional projection of the trioctylphosphine oxide supercell, with the C atoms shown in blue, H atoms in gray, P atoms in purple, and O atoms in red. The unit cell is outlined in black solid lines for clarity.
[Figure 3] Fig. 3. (a) Total X-ray scattering (black) is simulated as a function of crystallite size from 2 to 8 nm in diameter (green). The bulk crystal wide-angle diffraction calculation shows the most intense peak at 1.45 Å-1 corresponding to the (114) set of Miller planes followed by the characteristic peaks at q = 0.84, 1.61, 1.79, 1.95 Å-1 corresponding to the set of (200), (208), (117), (2,0,10) planes, respectively. (b) The Gaussian fits of the peaks for the 2 to 8 nm calculated scattering data has a linear dependence with the inverse of the full-width half-maximum values.
[Figure 4] Fig. 4. FT–IR spectroscopy of trioctylphosphine oxide show the characteristic vibrational stretching of the PO bond at 1146 cm-1, the P—C bond at 1465 cm-1, and C—H bonds from the alkyl chains at 2850 and 2919 cm-1.
[Figure 5] Fig. 5. Raman spectroscopy of trioctylphosphine oxide captures the vibrational modes for PO, P—C, and C—H stretching at 1145, 1138, and 2848/2882 cm-1, respectively.
Trioctylphosphine oxide top
Crystal data top
C24H51OPZ = 4
Mr = 386.62F(000) = 872
Monoclinic, P21/cDx = 0.984 Mg m3
a = 15.0889 (4) ÅMo Kα radiation, λ = 0.71073 Å
b = 5.2252 (1) ŵ = 0.12 mm1
c = 33.5535 (9) ÅT = 100 K
β = 99.449 (1)°Prism, colorless
V = 2609.55 (11) Å30.38 × 0.24 × 0.07 mm
Data collection top
Bruker APEXII
diffractometer
5926 independent reflections
Radiation source: sealed tube5115 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.026
phi and ω scansθmax = 27.5°, θmin = 1.7°
Absorption correction: multi-scan
(SADABS; Sheldrick, 2007)
h = 919
Tmin = 0.709, Tmax = 0.746k = 66
43246 measured reflectionsl = 4343
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.039H-atom parameters constrained
wR(F2) = 0.103 w = 1/[σ2(Fo2) + (0.0421P)2 + 1.0549P]
where P = (Fo2 + 2Fc2)/3
S = 1.10(Δ/σ)max = 0.001
5926 reflectionsΔρmax = 0.41 e Å3
239 parametersΔρmin = 0.35 e Å3
0 restraints
Crystal data top
C24H51OPV = 2609.55 (11) Å3
Mr = 386.62Z = 4
Monoclinic, P21/cMo Kα radiation
a = 15.0889 (4) ŵ = 0.12 mm1
b = 5.2252 (1) ÅT = 100 K
c = 33.5535 (9) Å0.38 × 0.24 × 0.07 mm
β = 99.449 (1)°
Data collection top
Bruker APEXII
diffractometer
5926 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 2007)
5115 reflections with I > 2σ(I)
Tmin = 0.709, Tmax = 0.746Rint = 0.026
43246 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0390 restraints
wR(F2) = 0.103H-atom parameters constrained
S = 1.10Δρmax = 0.41 e Å3
5926 reflectionsΔρmin = 0.35 e Å3
239 parameters
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
P10.964925 (19)0.28059 (5)0.807347 (8)0.01219 (8)
O10.97088 (6)0.55665 (15)0.79639 (2)0.01736 (18)
C11.06979 (7)0.1118 (2)0.80961 (3)0.0139 (2)
H1a1.06110.06670.81590.018*
H1b1.08880.11930.78340.018*
C21.14331 (8)0.2257 (2)0.84150 (3)0.0168 (2)
H2a1.15040.40520.83540.022*
H2b1.12400.21600.86760.022*
C31.23423 (8)0.0937 (2)0.84433 (3)0.0167 (2)
H3a1.22730.08740.84930.022*
H3b1.25580.11150.81880.022*
C41.30304 (8)0.2070 (2)0.87799 (4)0.0199 (2)
H4a1.31250.38510.87170.026*
H4b1.27820.20300.90290.026*
C51.39368 (8)0.0723 (3)0.88518 (4)0.0248 (3)
H5a1.38480.10630.89140.033*
H5b1.41970.07910.86060.033*
C61.45903 (9)0.1911 (3)0.91950 (5)0.0340 (3)
H6a1.46870.36860.91290.045*
H6b1.43190.18900.94380.045*
C71.54930 (11)0.0577 (4)0.92834 (6)0.0550 (5)
H7a1.57700.06010.90420.073*
H7b1.54010.11970.93510.073*
C81.61270 (15)0.1832 (6)0.96298 (9)0.0929 (10)
H8a1.62690.35340.95530.139*
H8b1.66690.08450.96890.139*
H8c1.58420.19050.98650.139*
C90.93336 (8)0.2526 (2)0.85700 (3)0.0147 (2)
H9a0.97500.35520.87560.020*
H9b0.87430.32850.85580.020*
C100.93063 (8)0.0148 (2)0.87519 (3)0.0159 (2)
H10a0.88770.11950.85760.021*
H10b0.98930.09410.87700.021*
C110.90403 (8)0.0038 (2)0.91718 (3)0.0179 (2)
H11a0.94690.10320.93440.024*
H11b0.84560.07730.91500.024*
C120.89991 (8)0.2624 (2)0.93744 (3)0.0163 (2)
H12a0.95810.34500.93950.022*
H12b0.85630.36930.92060.022*
C130.87432 (9)0.2429 (2)0.97944 (3)0.0198 (3)
H13a0.92050.14710.99670.026*
H13b0.81870.14740.97750.026*
C140.86248 (8)0.5003 (2)0.99905 (3)0.0176 (2)
H14a0.91820.59591.00120.023*
H14b0.81630.59670.98180.023*
C150.83663 (10)0.4757 (2)1.04085 (4)0.0264 (3)
H15a0.88370.38321.05820.035*
H15b0.78200.37551.03880.035*
C160.82193 (11)0.7311 (3)1.06044 (4)0.0302 (3)
H16a0.77370.82111.04410.045*
H16b0.80700.70221.08680.045*
H16c0.87590.83131.06280.045*
C170.88283 (7)0.1070 (2)0.77284 (3)0.0141 (2)
H17a0.90130.10360.74650.019*
H17b0.88070.06830.78210.019*
C180.78867 (8)0.2239 (2)0.76869 (3)0.0171 (2)
H18a0.76840.21580.79460.023*
H18b0.79170.40280.76130.023*
C190.72044 (8)0.0881 (2)0.73714 (3)0.0178 (2)
H19a0.74170.09210.71140.024*
H19b0.71640.08980.74490.024*
C200.62707 (8)0.2075 (2)0.73202 (4)0.0196 (2)
H20a0.60530.19970.75760.026*
H20b0.63150.38660.72490.026*
C210.55913 (8)0.0772 (2)0.69986 (4)0.0210 (3)
H21a0.55330.10050.70740.028*
H21b0.58170.08020.67440.028*
C220.46664 (8)0.2019 (3)0.69385 (4)0.0225 (3)
H22a0.44350.19580.71910.030*
H22b0.47260.38050.68690.030*
C230.39931 (9)0.0748 (3)0.66114 (4)0.0294 (3)
H23a0.39200.10270.66840.039*
H23b0.42280.07760.63590.039*
C240.30764 (9)0.2059 (3)0.65488 (5)0.0385 (4)
H24a0.28370.20220.67970.058*
H24b0.26750.11760.63420.058*
H24c0.31410.38030.64680.058*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
P10.01648 (14)0.00943 (14)0.01102 (14)0.00030 (11)0.00335 (10)0.00056 (10)
O10.0257 (4)0.0109 (4)0.0163 (4)0.0000 (3)0.0058 (3)0.0012 (3)
C10.0173 (5)0.0121 (5)0.0126 (5)0.0005 (4)0.0034 (4)0.0004 (4)
C20.0170 (5)0.0167 (6)0.0169 (5)0.0007 (4)0.0033 (4)0.0026 (4)
C30.0188 (6)0.0167 (6)0.0151 (5)0.0004 (4)0.0044 (4)0.0001 (4)
C40.0170 (6)0.0221 (6)0.0206 (6)0.0002 (5)0.0034 (4)0.0023 (5)
C50.0191 (6)0.0262 (7)0.0285 (7)0.0026 (5)0.0022 (5)0.0007 (5)
C60.0203 (6)0.0389 (8)0.0398 (8)0.0019 (6)0.0037 (6)0.0038 (7)
C70.0250 (8)0.0568 (12)0.0756 (13)0.0091 (8)0.0143 (8)0.0050 (10)
C80.0389 (11)0.100 (2)0.121 (2)0.0108 (12)0.0427 (13)0.0212 (17)
C90.0186 (5)0.0132 (5)0.0128 (5)0.0013 (4)0.0042 (4)0.0002 (4)
C100.0209 (6)0.0147 (5)0.0130 (5)0.0001 (4)0.0050 (4)0.0005 (4)
C110.0247 (6)0.0166 (6)0.0134 (5)0.0010 (5)0.0065 (4)0.0014 (4)
C120.0208 (6)0.0158 (6)0.0131 (5)0.0003 (4)0.0049 (4)0.0009 (4)
C130.0295 (6)0.0159 (6)0.0157 (5)0.0013 (5)0.0088 (5)0.0019 (4)
C140.0230 (6)0.0164 (6)0.0143 (5)0.0007 (5)0.0059 (4)0.0008 (4)
C150.0447 (8)0.0195 (6)0.0187 (6)0.0017 (6)0.0158 (6)0.0021 (5)
C160.0481 (9)0.0243 (7)0.0215 (6)0.0031 (6)0.0157 (6)0.0049 (5)
C170.0173 (5)0.0124 (5)0.0125 (5)0.0011 (4)0.0027 (4)0.0004 (4)
C180.0179 (5)0.0161 (6)0.0173 (5)0.0019 (4)0.0029 (4)0.0023 (4)
C190.0188 (6)0.0176 (6)0.0166 (5)0.0016 (5)0.0021 (4)0.0012 (4)
C200.0181 (6)0.0207 (6)0.0198 (6)0.0009 (5)0.0029 (4)0.0018 (5)
C210.0195 (6)0.0229 (6)0.0199 (6)0.0011 (5)0.0011 (5)0.0013 (5)
C220.0186 (6)0.0250 (6)0.0236 (6)0.0001 (5)0.0024 (5)0.0007 (5)
C230.0219 (6)0.0379 (8)0.0266 (7)0.0015 (6)0.0014 (5)0.0006 (6)
C240.0207 (7)0.0525 (10)0.0397 (8)0.0011 (7)0.0034 (6)0.0061 (7)
Geometric parameters (Å, º) top
P1—O11.4949 (8)C11—C121.5184 (16)
P1—C171.7962 (11)C12—C131.5246 (15)
P1—C11.8023 (11)C13—C141.5207 (16)
P1—C91.8123 (11)C14—C151.5217 (16)
C1—C21.5300 (15)C15—C161.5196 (18)
C2—C31.5246 (16)C17—C181.5316 (15)
C3—C41.5229 (16)C18—C191.5251 (16)
C4—C51.5217 (17)C19—C201.5247 (16)
C5—C61.5202 (18)C20—C211.5217 (16)
C6—C71.516 (2)C21—C221.5232 (17)
C7—C81.527 (3)C22—C231.5201 (18)
C9—C101.5283 (15)C23—C241.527 (2)
C10—C111.5281 (15)
O1—P1—C17113.05 (5)C11—C10—C9111.23 (9)
O1—P1—C1113.26 (5)C12—C11—C10114.54 (10)
C17—P1—C1106.72 (5)C11—C12—C13112.92 (9)
O1—P1—C9109.84 (5)C14—C13—C12113.99 (10)
C17—P1—C9107.12 (5)C13—C14—C15112.97 (10)
C1—P1—C9106.46 (5)C16—C15—C14113.71 (11)
C2—C1—P1111.70 (8)C18—C17—P1112.87 (8)
C3—C2—C1114.16 (9)C19—C18—C17112.84 (9)
C4—C3—C2111.54 (9)C20—C19—C18113.19 (10)
C5—C4—C3115.04 (10)C21—C20—C19113.53 (10)
C6—C5—C4112.68 (11)C20—C21—C22113.48 (10)
C7—C6—C5114.25 (14)C23—C22—C21113.46 (11)
C6—C7—C8112.46 (17)C22—C23—C24112.74 (12)
C10—C9—P1117.98 (8)

Experimental details

Crystal data
Chemical formulaC24H51OP
Mr386.62
Crystal system, space groupMonoclinic, P21/c
Temperature (K)100
a, b, c (Å)15.0889 (4), 5.2252 (1), 33.5535 (9)
β (°) 99.449 (1)
V3)2609.55 (11)
Z4
Radiation typeMo Kα
µ (mm1)0.12
Crystal size (mm)0.38 × 0.24 × 0.07
Data collection
DiffractometerBruker APEXII
diffractometer
Absorption correctionMulti-scan
(SADABS; Sheldrick, 2007)
Tmin, Tmax0.709, 0.746
No. of measured, independent and
observed [I > 2σ(I)] reflections
43246, 5926, 5115
Rint0.026
(sin θ/λ)max1)0.651
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.039, 0.103, 1.10
No. of reflections5926
No. of parameters239
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.41, 0.35

Computer programs: APEX2 (Bruker, 2009), SAINT (Bruker, 2009), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), ORTEPII (Johnson, 1976).

 

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