Molecular and Supramolecular Origins of Optical Nonlinearity in

Article pubs.acs.org/JPCC

Molecular and Supramolecular Origins of Optical Nonlinearity in N‑Methylurea Jacqueline M. Cole,*,†,‡ Paul G. Waddell,†,‡ Chick C. Wilson,§ and Judith A. K. Howard∥ †

Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom Department of Chemistry, University of New Brunswick, P.O. Box 4400, Fredericton NB E3B 5A3, Canada § Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom ∥ Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, United Kingdom ‡

S Supporting Information *

ABSTRACT: The delicate balance between solid-state intermolecular interactions and electron-donating methyl-group influences in N-methylurea (NMU) is shown to distinguish its nonlinear optical properties, relative to those of urea, a standard reference material for second harmonic generation (SHG). The solid-state intermolecular interactions in NMU are identified using neutron diffraction data, showing that hydrogen bonding generates an extensive 3D supramolecular network of NMU molecules with secondary and tertiary nonbonded contacts helping to hold this network in a closely packed form. The undulating “urea tape” motif within this network renders an overall packing arrangement that is less SHG-favorable than that of urea, which exhibits a more head-to-tail molecular alignment. The primary, secondary, and tertiary nonbonded contacts are classified using graph-sets, Hirshfeld surfaces, and fingerprint plots. H···H contacts in NMU contribute to the overall Hirshfeld surface area much more than in urea, forming at the expense of O···H interactions. However, SHGcontributing electronic effects of the methyl group in NMU provide some compensation to these hydrogen-bonding influences. This methyl group is also shown to librate, which could augment SHG. Our experimental results offer a direct response to previous density functional theory calculations on NMU and urea,1 corroborating their predictions as well as enabling a better relationship between the molecular and bulk optical nonlinearity of NMU. To that end, crystal engineering options are discussed with a view to balancing these seemingly conflicting structural attributes, so that one can produce an SHG-active form of NMU that is superior to urea.

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INTRODUCTION

Accordingly, NMU has been investigated in terms of its thermal properties, including measurements of its thermal conductivity, 22 heat capacity,22,23 standard enthalpy of combustion, sublimation enthalpy, and its vapor pressure relative to liquid NMU.23 Given this extensive characterization of the thermal and nonlinear optical properties of NMU, it is perhaps surprising that very little structural work has been carried out on this compound. This is all the more surprising given the well-known structure−property relationships in organic NLO materials.24 The basic X-ray derived crystal structure of NMU was first reported in 1933.10 However, the authors at the time suggested that the reported structure might be incorrect, and in 1976 Huiszoon and Tiemessen reported the correct three-dimensional structure.25 They also showed that three O···H−N hydrogen-bonds of moderate strength were present in the lattice. The likely governance of structure on the NLO properties of NMU, including the nature of these intermolecular interactions, prompted the present study; an additional

As the most commonly used standard reference compound for second harmonic generation (SHG), urea has long been an important material within the nonlinear optical industry. As a result, the thermal2−5 and nonlinear optical6 properties of urea have been studied extensively. Because of its industrial importance and the relative simplicity of the urea molecule, the three-dimensional structure of urea has also been studied extensively via X-ray7−16 and neutron diffraction,17,18 with these studies concentrating on hydrogen-bonding patterns,11,17 thermal motion analysis13,14,18 and charge density analysis.11,16 Nonlinear optical investigations have shown6 that Nmethylurea (hereafter NMU), the first monoalkyl derivative of urea, possesses a similar second-order optical nonlinearity and optical damage threshold to urea. Birefringent studies have also been performed yielding refractive index coefficients which show that the compound is phase-matchable.19 These similarities, in addition to the increased thermal stability, higher melting point and less hygroscopic nature of NMU compared to urea, have led researchers to propose NMU as a possible alternative to the established urea nonlinear optical standard.20,21 © 2013 American Chemical Society

Received: September 4, 2013 Revised: October 5, 2013 Published: October 15, 2013 25669

dx.doi.org/10.1021/jp4088699 | J. Phys. Chem. C 2013, 117, 25669−25676

The Journal of Physical Chemistry C

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Table 1. Summary of Crystal, Data Collection and Refinement Parameters for the 100−250 K Neutron Structures of NMU compound

100 K

225 K

molecular formula formula weight a (Å)a b (Å)a c (Å)a cell volume (Å3) crystal system space group Z calculated density (gcm−1) wavelength crystal size (mm) total number of reflections unique reflections observed reflections [I > 2σ (I)] Rint data/parameters R1, wR2 [I > 2σ (I)] goodness of fit on F2 weighting scheme Δρ(max, min) (fm Å−3)

C2H6N2O 74.00 8.555(2) 6.796(1) 6.793(3) 394.9(2) orthorhombic P212121 4 1.245 white beam 4.0 × 2.0 × 1.5 8734 1460 1439 0.0608 1439/100 0.0683, 0.1219 1.068 0.06020, 2.97840 1.615/−1.929

C2H6N2O 74.00 8.493(4) 6.999(4) 6.884(3) 409.2(4) orthorhombic P212121 4 1.201 white beam 4.0 × 2.0 × 1.5 671 671 671 0.0763 671/100 0.0780, 0.1686 1.317 0.03510, 7.50730 1.348/ −1.041

250 K

275 K

300 K

C2H6N2O 74.00 8.482(4) 6.923(4) 6.898(3) 405.1(3) orthorhombic P212121 4 1.213 white beam 4.0 × 2.0 × 1.5 2675 437 436



436/94 0.0622, 0.1540 1.176 0.087100, 3.539200 0.920/−0.990

521/100 0.0578, 0.1411 1.167 0.061200, 4.303200 0.973/ −0.704

369/100 0.0548, 0.1412 1.194 0.089700, 2.811900 0.597/ −0.723

a These values are taken from single-crystal X-ray diffraction unit cell determinations since they are deemed to be more accurate than those determined from neutron measurements.

the molecule to be assessed; in particular, methyl group libration is identified, which has implications for complementary density functional theory (DFT) calculations on NMU,1 to which our study is compared. In general, our results corroborate well these previous DFT calculations and afford further detail on the molecular and supramolecular origins of SHG in NMU. To this end, the molecular and bulk optical nonlinearity can be better related. On the basis of these results, a mechanism is herein proposed by which one can apply crystal engineering to optimize the bulk SHG output of NMU for its molecular-scale attributes.

consideration was the general lack of crystallographic data for NMU given the high industrial relevance of its parent compound, urea, within the NLO community. Intermolecular interactions have been shown to significantly influence the SHG properties of organic NLO materials.26−29 In essence, this is because SHG has an optical coherence length of μm and so is a bulk property, as measured by the secondorder susceptibility, χ(2); yet, SHG is typically engineered at the molecular level since a molecule must exhibit a significant first molecular hyperpolarizability, βijk, in order that SHG has the chance to propagate to the bulk. Sufficiently strong local-field factors,30 including electron correlation effects,31 are required to forge this “molecule-to-crystal” link and these are manifest via crystal field forces, to which various types of intermolecular interactions pertain. In this paper, we describe fully the extent of hydrogen bonding within NMU, accessed via neutron diffraction and use Hirshfeld surfaces and graph set notation to identify and classify these interactions with a view to linking this supramolecular chemistry to the thermal and nonlinear optical properties of NMU. The same intermolecular interaction classification methods were then used to compare supramolecular structural patterns between NMU and the parent NLO crystal, urea. To this end, the primary crystal field forces that dictate the optical nonlinearity in these two materials could be assessed in a relative manner. Neutron diffraction was the selected atomic probe for this study since this is the most accurate technique for determining the positions of hydrogen atoms. Given past focus on the thermal properties of NMU as a physical property of potential relevance to its value as an applied material, the neutronderived structure of NMU is determined at five different temperatures so that any temperature-dependent effects on hydrogen bonding could be investigated. The multi-temperature nature of this study also enabled the dynamic aspects of

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EXPERIMENTAL SECTION The neutron structure of NMU was determined using data collected on SXD at the ISIS facility, Rutherford Appleton Laboratory, U.K.32 Five sets of data were collected at 100, 225, 250, 275, and 300 K (Table 1). The 100 K data-set collection was more extensive than that of the others on account of the superior accuracy obtainable in its bond geometry owing to the reduced thermal smearing effects at this temperature. A crystal of dimensions 4.0 × 2.0 × 1.5 mm was cut from a parent crystal (42 × 8 × 7 mm), mounted on an aluminum pin and fixed onto the cold-head of a Displex cryorefrigerator. Data were collected using standard SXD procedures at the time.33 Peaks from initial frames at room temperature were strong and exhibited good profiles. Consequently, the sample was successfully indexed using a frame of data collected for only a few microampere-hours. The sample was then cooled to 100 K and the data collection initiated. Up to the point of tiling, where frames overlap in order to gain data redundancy, each frame was collected for 500 μA·h. When tiling began, this was reduced to 250 μA·h. ϕ and χ were moved in increments of 40 and 30° respectively, starting at ϕ = 0°, χ = 0° initially and starting at ϕ = 20°, χ = 75° when the tiling process began. The other sets of data followed a similar data collection procedure to that used for the 100 K data collection except that 25670



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each frame was collected for only 150 μA·h up to the point of tiling, whereupon it was reduced to 100 μA·h. For the 250 K data collection, a second ϕ = 0, χ = 0 frame was additionally collected at the end of the experiment in order to improve the statistics. Standard SXD procedures were used to process and correct all data sets.33 Subsequent structural refinements were carried out using SHELXL-97 (Figure 1).34 Cell parameters for each refinement were interpolated from previous multi-temperature unit cell determinations that employed X-ray diffraction22 since such parameters are more reliable than those derived from a neutron source. All atomic positions and anisotropic displacement parameters were refined independently. Libration was observed in the hydrogen atoms of the methyl group, manifest by the highly ellipsoidal nature of the ADPs on the methyl hydrogen atoms that increase with temperature (Figure 1) and a corresponding foreshortening of the CMethyl−H bond distances within the structure. A translation-libration-screw (TLS) analysis35 was successfully performed yielding longer CMethyl−H bond distances and a quantifiable temperature dependence (see Supporting Information), as one would expect for libration. Regarding a putative phase transition in NMU, as suggested by others,22 no evidence of this was detected in the neutronderived molecular geometry of NMU up to 300 K; although, significant condensation issues were encountered upon warming the crystal from 275 to 300 K in order to collect the 300 K data, given the hygroscopic nature of NMU. Complementary X-ray data collections at various temperatures (see Supporting Information) were also unable to detect a phase transition. These X-ray crystal structure data were collected using a Rigaku Saturn 724+ CCD diffractometer that houses a molybdenum X-ray source (λMo, Kα = 0.71073 Å) with SHINE Optics and an Oxford Cryostreams CryostreamPlus open-flow N2 cooling device. Cell refinement, data collection, and data reduction were undertaken via the Rigaku CrystalClear-SM Expert 2.0 software.36 The absorption correction was implemented using ABSCOR.37 The X-ray derived crystal structures were solved by direct methods and refined by full-matrix least-squares methods on F2 values of all data. Refinements were performed using SHELXL.34 Hydrogen atoms were positioned geometrically and refined as riding on their parent atoms. The neutron derived nonhydrogen molecular geometry is very similar to the X-ray derived geometry.25 Moreover, within error, there appears to be no significant variation in this geometry with temperature. The hydrogen positions determined in this study are naturally more accurate than those reported by Huiszoon and Tiemessen25 as well as being longer due to foreshortening of X-H bonds in X-ray diffraction. The overall accuracy of the 225, 250, 275, and 300 K neutronderived structural determinations of NMU is necessarily lower than that of the 100 K structural determination, given the much smaller amount of data acquisition time for the higher temperature data sets. Therefore, our hydrogen-bonding studies concentrated primarily on the 100 K neutron-derived structure, although an analysis at all temperatures is reported.

Figure 1. Fifty percent probability ellipsoid plots of structure of NMU at multiple temperatures.

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RESULTS AND DISCUSSION 1.1. Characterizing Intermolecular Interactions in NMethylurea: Graph Set Analysis. All data show clearly that the three-dimensional lattice arrangement of NMU is dictated

by hydrogen bonding (Table 2). The elements that make up this infinite hydrogen-bonding net are best described with the use of Etter-Bernstein notation,38,39 where hydrogen-bonding motifs are described in terms of graph sets. The three25671



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Table 2. Hydrogen Bond Geometry (Å, °) for NMU at 100 K (See Supporting Information for Hydrogen Bond Geometry at Other Subject Temperatures)a A···H−D O(1)···H(2)−N(1)i O(1)···H(3)−N(2)ii O(1)···H(1)−N(1)ii C−H···X C(2)−H(4)···C(1)iii C(2)−H(4)···N(1)iii C(2)−H(5)···C(1)iv C(2)−H(6)···O(1)v

D−H 1.013(3) 1.016(3) 1.007(3) C−H 1.056(6) 1.056(6) 1.079(7) 1.079(5)

H···A 1.942(4) 1.967(4) 2.042(4) H···X 2.877(8) 2.683(8) 2.771(8) 2.593(6)

D···A 2.9469(18) 2.9109(18) 2.9524(17) C···X 3.797(2) 3.712(2) 3.593(2) 3.569(2)

and urea itself.7−16 All other N-alkylurea structures exhibit twodimensional hydrogen-bonding networks and, with the exception of N-butylurea, additional R22(8) rings that are absent from the structures of urea and NMU. These R22(8) rings link two antiparallel R12(6) urea tape motifs (the C(4)C(4)[R12(6)] graph set is common to all N-alkylureas). It should be noted that the urea tape motif in NMU forms an undulating chain with a ca. 49° angle between the planes of adjacent urea moieties, whereas the urea tape in urea itself forms a chain of coplanar units. While these hydrogen bonds can be considered to be the primary nonbonded contacts affecting the three-dimensional arrangement of the molecule, all secondary nonbonded contacts were also investigated, using Fingerprint plots generated from Crystal Explorer, version 2.1.43 This not only enabled a more detailed understanding of the intermolecular interactions ensuing in the crystal lattice, it also provided pattern-based classification by which intermolecular interactions in NMU and urea could be compared. 1.2. Secondary and Tertiary Nonbonded Interactions: Hirshfeld Surface Analysis. In addition to the primary nonbonded contacts, which represent the classical D−H···A hydrogen bonds, the presence of the methyl group in NMU allows for the formation of secondary nonbonded contacts, defined here as the weaker hydrogen bonding of the type C− H···X.44 In order to better investigate these more subtle secondary intermolecular interactions, Hirshfeld surfaces have been calculated for NMU using CrystalExplorer 2.1.43 Hirshfeld surfaces are representative of the electron distribution within the crystal and are calculated as the sum of the electron densities of isotropic atoms. Identification of close contacts is made possible via the normalized contact distance (dnorm) relative to the distances from the surface to the nearest nucleus inside and outside the surface (di and de respectively) given by eq 1. Close intermolecular contacts, that is, those closer than the sum of van der Waals, are indicated by the corresponding red areas on the Hirshfeld surfaces of the molecules.

D−H···A 170.7(3) 153.3(3) 148.8(3) C−H 145.7(5) 164.6(6) 132.9(6) 150.0(5)

Symmetry codes: (i) 0.5 − x, 2 − y, 0.5 + z; (ii) 0.5 + x, 1.5 − y, 1 − z; (iii) 1 − x, −0.5 + y, 0.5 − z; (iv) 0.5 − x, 1 − y, −0.5 + z; (v) 0.5 − x, 2 − y, −0.5 + z. a

dimensional hydrogen-bonding network in 1 forms through the presence of three chain motifs, each corresponding to the screw axes along the [100], [010] and [001] directions respectively. The simplest of these is the C(4) motif along [001] (through O(1)···H(2)−N(1), Figure 2). The remaining two motifs both consist of chains of rings. These rings are described by the graph set R12(6) and are formed via bifurcated hydrogenbonding between the carbonyl oxygen of one molecule and two nitrogen-bound hydrogens on another (O(1)···H(1)−N(1) and O(1)···H(3)−N(2)). The rings combine along [100] to give the typical “urea tape” motif:40 a chain of rings with the graph set C(4)C(4)[R12(6)] (Figure 2) which, together with the C(4) motif along [001], forms infinite two-dimensional sheets. The three-dimensional network is completed by the second chain of rings, which forms along the [010] and has the graph set C22(8)C22(8)[R12(6)] (Figure 2). It is possible to identify other hydrogen-bonding motifs within the structure, such as the R56(20)R56(22)[R12(6)] rings that lie parallel to the (011) plane or the C34(12)C34(14)[R12(6)] chain along [111] but these all occur as corollary of the three interlocking hydrogen-bonding chains on each of the three screw axes described above. Considering all structurally characterized N-alkylureas (NH2C(O)NH(CxH2x+1) where x = 1−14),25,41,42 the threedimensional packing motif observed here is unique to NMU

dnorm =

d i − r ivdW rivdW

+

de − revdW revdW

(1)

Figure 2. (Left to Right): The C(4)C(4)[R12(6)], C22(8)C22(8)[R12(6)], and C(4) motifs in the neutron structure of NMU. 25672



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Figure 3. Two orientations of the Hirshfeld surface of NMU at 100 K with significant nonbonded contacts highlighted.

Figure 4. Fingerprint plots for NMU (left) and Urea (right). Those for NMU at 225, 250, 275, and 300 K are available in the Supporting Information.

“Fingerprint” plots of each Hirshfeld surface were also derived from CrystalExplorer 2.1 by plotting di against de, providing a more visual description of this topological characteristic. Breaking the surfaces down into percentage areas of the surface associated with specific interactions allows “barcode” figures for each molecule to be produced, further facilitating this characterization. Hirshfeld surfaces, fingerprint plots and barcode figures have primarily been used to characterize hydrogen-bonding systems45−48 but more recently we have demonstrated their usefulness in determining less obvious intermolecular interactions,49 which makes them ideal for investigating these secondary contacts. The largest and most noticeable red areas on the Hirshfeld surface of NMU (Figure 3) correspond, as expected, to the primary O···H hydrogen-bonding interactions. The smaller, paler spots observed on the surface are due to the secondary close contacts which all involve the methyl hydrogen atoms (Table 2). The interaction between a methyl hydrogen H(6) and O(1) appears to be the strongest of these interactions given its short H···X distance. The close contact between H(5) and C(1)

could potentially be the result of sterics as opposed to a genuine electrostatic interaction as evidenced by the relatively low C−H···X angle. The methyl hydrogen H(4) is involved in a bifurcated interaction with both C(1) and N(1), which is manifest as two overlapping red spots on the Hirshfeld surface. Once again the low C−H···X angle suggests that this C···H may also be a steric effect. The barcode graphs reveal the extent of the tertiary nonbonded contacts. While the primary O···H hydrogen bonds and secondary C−H···X are strong enough interactions to show up as red spots on the Hirshfeld surface, together they account for less than 50% of the surface. The remainder of the surface can be attributed to very weak tertiary H···H contacts; however, as these are likely to be repulsive and in any case are not within the range of the sum of van der Waals radii, they are not likely to be structure-directing and will therefore have little effect on the properties of the materials. 2. Comparing Intermolecular−Interaction Patterns in NMU and Urea. These fingerprint plots (Figure 4) and barcode graphs can be readily employed to compare the supramolecular structure of NMU with that of urea. In this 25673



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intraband transitions. The associated band structure enabled a decomposition of NLO contributions into individual bonding states. To that end, specific molecular groups or supramolecular interactions (manifested via polarized valence electrons) that particularly impact on SHG could be identified by comparing common and disparate band structure traits between urea, methylurea and dimethylurea. In this regard, the importance of the hydrogen-bonding network was highlighted in that study since the SHG in methylureas was predicted to originate as a result of the strong push−pull effects observed along the hydrogen bonds, allowing charge transfer in the direction of the urea tape motif. On the one hand, this suggests that the undulating urea tape motif in NMU is less well-suited for NLO purposes than urea, which exhibits a more direct head-to-tail alignment. On the other hand, the theoretical study also indicates that the presence of the electron donating methyl group in NMU has a significant NLO-enhancing role to play. At the molecular level, the methyl group yields more electron-rich acceptor atoms which, in turn, produce larger molecular dipole moments. The propagation of such moments to the supramolecular level is nonetheless dictated by the nature of the crystal field forces of which hydrogen-bonding interactions dominate. However, density-of-states calculations by Luo et al.1 indicate that SHG contributions from the methyl group are carried through to the bulk state; distinct features in the absorptive part of the dielectric function in their real and virtual states, ε″(ω) and ε″(ω/2), were predicted in NMU but not in urea, and, when related to the χ″(2) peak at 4.7 eV, these features could be attributed to the C 2p and H 1s states of the CH3 group. Corresponding analysis at double-photon resonances further suggest that the influence of the methyl group on the SHG phenomenon has its electronic origins primarily in the virtual hole process, which enhances SHG. It is very pertinent to note that those calculations had no notion of the methyl group libration revealed by this study; yet, its modeling could alter the calculated electronic transitions considerably. This libration therefore stands to affect the SHG properties of NMU significantly. Indeed, SHG enhancement, owing to the libration of a molecular fragment, has previously been proposed with regards to another organic SHG-active material.50 Given these apparent relationships, it would therefore seem optimal to find a way to strike a good balance between the methyl-group assets of NMU for SHG against the compromise in the nature of its hydrogen-bonding network relative to that of urea. Crystal engineering via the judicious cocrystallization of NMU with another small, hydrogen-bond donor rich moiety could be a way forward; the strategy being to retain the methyl group SHG characteristics, while creating a supramolecular hydrogen-bonding network that is richer in O···H contacts and manifests a pattern that is more akin to the SHG-favored urea framework. While such crystal engineering objectives present a serious challenge to current research efforts, the use of cocrystallization to control optical properties has already been demonstrated in other areas of optoelectronics51 and there are some obvious starting points for cocrystallization in the subject study, for example, cocrystallizing NMU with other Nalkylureas, urea, or water (given NMU is hygroscopic). This will be the subject of further work.

study, the 100 K neutron data for NMU is compared to the 123 K neutron structure of urea reported by Swaminathan et al. since this report represents the closest study in terms of temperature to the subject NMU study, and it is also the most accurate of the two 123 K published data sets of urea, as judged by its superior R-factor and the lowest unit cell errors.18 Regarding the fingerprint plots, the two “spikes” of the NMU fingerprint appear distinctly longer and sharper than those of urea; this corresponds to the shorter hydrogen bonds of the urea tape formation in NMU. An overall increase in the number of H···H contacts in the Hirshfeld surface of NMU, compared to urea, results from the presence of the methyl group in NMU; this difference is reflected in the increased coverage of larger distances in the fingerprint plot (around de = di = 2.2 Å) as well as a flattening and widening of the areas around the base of the spikes. This large increase in the number of H···H interactions in NMU, relative to urea, can also be seen on the barcode graphs (Figure 5), where it is also evident that this increase

Figure 5. Barcode graphs for NMU and urea.

comes at the cost of a diminished Hirshfeld surface contribution from corresponding O···H and H···O interactions. This lower O···H Hirshfeld surface area can be traced back to the classical hydrogen-bond enquiry (see Section 2) as there are eight O···H hydrogen bonds associated with one molecule of urea but only six in NMU. The addition of the methyl group is again the cause of this disparity since it replaces an otherwise possible hydrogen-bond donor site. 3. Relationships between Supramolecular Structure of NMU and Its Nonlinear Optical Properties. Some links between the crystal structure and supramolecular chemistry of alkyl urea derivatives and their optical properties have previously been established. For example, the magnitude of refractive index coefficients in NMU have been related to the orientation of the molecules in the crystal with the direction in which the carbonyl bonds are aligned yielding the largest linear optical properties as this is likely to be the most polarizable direction.19 The recent theoretical work of Luo et al. has provided some important insights that relate the structure of N-alkylureas to their nonlinear optical properties.1 Therein, the dielectric constants, ε(ω), and second-order nonlinear optical susceptibilities, χ(2), were calculated for a range of methylurea derivatives using density functional theory. Within this scope, real and imaginary components of both ε and χ(2) were realized for N-methylurea, within a wide frequency range (0−10 eV); as such, one could account for contributions to ε and χ(2) across a wide density of states, thereby incorporating the effects of many

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CONCLUDING REMARKS The supramolecular structure and molecular dynamics in NMU have been established and related to their optical properties in 25674



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comparison to those of urea. The fact that these experimental findings corroborate well previous DFT studies on NMU and urea is highly encouraging.1 While the viability of replacing urea for NMU as a standard reference for SHG is still not established, the results of this work can only help this prospect, by providing structure−property relationships that can act as smart material design strategies for incorporating NMU into an SHG-optimal cocrystallized environment. Given that NMU is such a small organic molecule, its supramolecular structure will be particularly influential toward its SHG output. However, the nature by which its solid-state intermolecular interactions have been classified here and compared to those of urea is generally significant. In particular, the classification of all nonbonded contacts via fingerprint and barcode pattern analyses of its Hirshfeld surface, as presented in this study, can be readily, and very helpfully, translated to small or large-scale comparison studies of other organic NLO materials where supramolecular structure is an important consideration. With crystal engineering at the forefront of current materialsby-design efforts, the type of structure−property relationships established and compared herein can be considered as an individual specification of what will likely become a far larger and highly automated systematic effort in materials discovery; the basis of these analyses can be scaled up not only to discover more optimal SHG-active materials but any material whose properties require a tailoring of supramolecular chemistry.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

More detailed THMA11 output for N-methylurea neutron diffraction data is deposited according to its orthogonal coordinates of TLS-corrected atoms, librational tensors at all studied temperatures, librational corrections and associated modified bond-lengths at all studied temperatures, plots of L and T tensors as a function of temperature. Graphs of U11, U22, and U33 for all N-methylurea atoms as a function of temperature are also given. Bond lengths, bond angles, hydrogen-bond geometry, and secondary nonbonded contact geometry for N-methylurea at all studied temperatures are also deposited, together with a full temperature decomposition of fingerprint plots and barcode graphs. This material is available free of charge via the Internet at http://pubs.acs.org.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the EPSRC for access to neutron beam-time at the ISIS facility, Rutherford Appleton Laboratory, Chilton, U.K., and Professor John Sherwood and Evelyn Shepherd, formerly from the University of Strathclyde, for supplying the crystals. J.M.C. is indebted to the Institut Laue Langevin, Grenoble, France, for financial support, the Royal Society for a University Research Fellowship, UNB for The Vice-Chancellor’s Research Chair, and NSERC for Discovery Grant, 355708 (for P.G.W.). 25675



Article

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