Ternary Copper(II) Coordination Compounds with Nonpolar Amino Acids and 2,2 ′ -Bipyridine: Monomers vs. Polymers

: Reactions of copper(II) sulfate with 2,2 ′ -bipyridine (bipy) and amino acids with nonpolar side chains ( L -alanine (HAla), L -valine (HVal), or L -phenylalanine (HPhe)) were investigated under different solution-based and mechanochemical methods. Five new ternary coordination compounds were obtained by a solution-based synthesis and three of them additionally by the liquid-assisted mechanochemical method: {[Cu( µ - L -Ala)(H 2 O)(bipy)] 2 SO 4 · 2H 2 O} n ( 1a · 2H 2 O ), {[Cu( µ - L - Ala)(H 2 O)(bipy)][Cu( L -Ala)(H 2 O)(bipy)]SO 4 · 2.5H 2 O} n ( 1b · 2.5H 2 O ), {[Cu( µ - L -Val)(H 2 O)(bipy)][Cu( L - Val)(H 2 O)(bipy)] 3 (SO 4 ) 2 · 4H 2 O} n ( 2 · 4H 2 O ), [Cu( L -Phe)(H 2 O)(bipy)][Cu( L -Phe)(SO 4 )(bipy)] · 8H 2 O ( 3 · 8H 2 O ), and [Cu( L -Phe)(H 2 O)(bipy)][Cu( L -Phe)(SO 4 )(bipy)] · 9H 2 O ( 3 · 9H 2 O ). The compounds were characterized by single-crystal and powder X-ray diffraction, infrared spectroscopy, and a thermal analysis. Structural studies revealed two structural types, monomeric in 3 · 8H 2 O and 3 · 9H 2 O , polymeric architectures in 1a · 2H 2 O , and mixed structures (monomeric and polymeric) in 1b · 2.5H 2 O and 2 · 4H 2 O . The copper(II) ion is either pentacoordinated or hexacoordinated, with an observed Jahn–Teller effect. The crystal structures are based on an intensive network of hydrogen bonds and π interactions. 1a · 2H 2 O and 2 · 4H 2 O showed substantial in vitro antiproliferative activity toward human hepatocellular carcinoma (HepG2) and moderate activity toward human acute monocytic leukemia cell lines (THP-1).

After the discovery of cisplatin [11], great attention has been given to the transition metal coordination compounds with a similar biological activity, especially to the ternary copper(II) compounds with amino acids and N,N'-donor heterocyclic bases such as 2,2 ′ -bipyridine or 1,10-phenanthroline derivatives.It was shown that the presence of heterocyclic bases, which act as N-N ligands, stabilizes the molecular structure and is crucial for antiproliferative activity [1].These compounds belong to the group of compounds called Casiopeinas, which stand out because of their antiproliferative properties and can be used as cytotoxic or diagnostic agents as well as antitumor or antiviral medicine [1,12,13].The antiproliferative properties arise from DNA binding abilities in the physiological

Materials and Methods
The copper(II) sulfate pentahydrate was purchased from Gram-mol; 2,2 ′ -bipyridine from Acros Organics; L-alanine, L-valine, and L-phenylalanine from Fisher Bioreagents; and methanol from Carlo Erba Reagents.The copper(II) hydroxide was prepared by a method described in the literature [33,34].A Retch MM200 ball mill was used for grinding experiments, working at a frequency of 25 Hz, with Teflon jars (volume of 14 mL) and stainless-steel grinding balls (diameter of 8 mm).For a thermogravimetric analysis, the Mettler Toledo TGA/DSC 3+ was used under an oxygen flow of 50 mL min −1 and a heating rate of 10 • C min −1 in the temperature range of 25-800 • C. The sample (approximately 8.3-15.5 mg) was placed in a standard alumina crucible (70 µL).IR(ATR) spectra were Crystals 2024, 14, 656 3 of 15 measured using a Thermo Scientific™ Nicolet™ iS50 FTIR Spectrometer in ATR mode in the range of 4000-400 cm −1 .
General procedure for solution-based syntheses.Copper(II) hydroxide (0.25 mmol); copper(II) sulfate pentahydrate (0.25 mmol); 2,2 ′ -bipyridine (0.5 mmol); amino acids (Lalanine, L-valine, or L-phenylalanine-0.5 mmol); and a solvent (water, methanol, or a mixture of water and methanol-10 mL) were mixed.The solution was heated until most of the reactants were dissolved.The reaction mixture was filtered off if some precipitate was left after approximately one hour.

Solution-based synthesis of
). L-valine (59.3 mg and 0.5 mmol), 2,2 ′ -bipyridine (78.4 mg and 0.5 mmol), copper(II) hydroxide (24.1 mg and 0.25 mmol), and copper(II) sulfate pentahydrate (63.1 mg and 0.25 mmol) were dissolved in water or methanol (10 mL) and heated for 45 min.The precipitate was filtered, and the dark blue filtrate was left to evaporate at room temperature for a few weeks until dark blue prismatic crystals of

Solution-based synthesis of
). L-phenylalanine (82.5 mg and 0.5 mmol), 2,2 ′ -bipyridine (78.1 mg and 0.5 mmol), copper(II) hydroxide (24.4 mg and 0.25 mmol), and copper(II) sulfate pentahydrate (62.4 mg and 0.25 mmol) were dissolved in a mixture of water and methanol (10 mL, 1:9 v/v) and heated for 15 min.The precipitate was filtered, and the dark blue filtrate was left to evaporate at room temperature for a few days until dark blue prismatic crystals of 3•8H 2 O formed.The crystals were analyzed by single-crystal X-ray diffraction.If water is used as the solvent, a glass-like solid is formed upon evaporation of the solvent.The crystallization of 3•8H 2 O is highly dependent on external conditions, so in some experiments, [Cu(µ-L-Phe) 2 ] n was formed instead [35].The crystal data for  ).L-phenylalanine (82.4 mg and 0.5 mmol), 2,2 ′ -bipyridine (77.9 mg and 0.5 mmol), copper(II) hydroxide (24.1 mg and 0.25 mmol), and copper(II) sulfate pentahydrate (62.4 mg and 0.25 mmol) dissolved in 10 mL of methanol, and the mixture was heated for 15 min.The precipitate was filtered, and the dark blue filtrate was left to evaporate at room temperature for a few days until dark blue prismatic crystals of 3•9H 2 O formed.Crystals were analyzed by single-crystal X-ray diffraction.The crystallization of 3•9H 2 O is highly dependent on external conditions, so in some cases, [Cu(µ-L-Phe) 2 ] n was formed instead [35] Liquid-assisted grinding (LAG) mechanochemical syntheses of 1b•2.5H 2 O. 2,2 ′bipyridine (78.2 mg and 0.5 mmol), copper(II) hydroxide (24.4 mg and 0.25 mmol), copper(II) sulfate pentahydrate (62.3 mg and 0.25 mmol), L-alanine (44.3 mg and 0.5 mmol), and methanol (41.8 µL and η = 0.2 µL mg −1 ) were placed in a Teflon jar with one stainless steel ball.Milling was carried out for 15 min at room temperature.The product was analyzed by powder X-ray diffraction, and the powder pattern was consistent with the pattern calculated from the single-crystal structure data of 1b•2.5H 2 O (Figure S1).
Liquid-assisted grinding (LAG) mechanochemical syntheses of 3•8H 2 O. 2,2 ′ -bipyridine (78.1 mg and 0.5 mmol), copper(II) hydroxide (24.4 mg and 0.25 mmol), copper(II) sulfate pentahydrate (62.4 mg and 0.25 mmol), L-phenylalanine (82.6 mg and 0.5 mmol), and water (49.5 µL and η = 0.2 µL mg −1 ) were placed in a Teflon jar with one stainless steel ball.Milling was carried out for 15 min at room temperature.The product was analyzed by powder X-ray diffraction, and the TGA was conducted.The powder pattern was mostly consistent with the pattern calculated from the single-crystal structure data of 3•8H 2 O (Figure S3).Some additional peaks appeared, probably due to the slow decomposition of the sample over time, as seen in the changes in the diffraction pattern over time (Figure S3).The TGA of the same sample gave a water fraction close to the theoretical value for 3•8H 2 O (exp., 15.6%; theor., 15.8%).
Single-crystal X-ray Diffraction.The Oxford Diffraction Xcalibur 2 CCD diffractometer was used for a single-crystal X-ray diffraction analysis of 1b•2.5H 2 O, with a graphite monochromator and MoKα source of radiation (λ = 0.71073 Å) by a ω scan at room temperature.The XtaLAB Synergy-S diffractometer, at a temperature of 170 K and with MoK α radiation (λ = 0.71073 Å), was used for the analysis of 1a•2H 2 O, 2•4H 2 O, and 3•9H 2 O, while CuK α radiation (λ = 1.54184Å) was used for the analysis of 3•8H 2 O.The collection and reduction of data were performed by the CrysAlis software package [36].Crystal structures were solved using direct methods by SHELXS [37] and refined by the SHELXL [38] program, incorporated within the WinGX program system [39].The structures were refined by the full-matrix least-squares method based on F 2 against all reflections.All non-hydrogen atoms were refined anisotropically.The hydrogen atoms of aminoacidates and 2,2 ′ -bipyridine ligands were found in the Fourier difference map, but due to the poor geometry of some of them, they were placed on calculated positions for the corresponding functional group.Hydrogen atoms belonging to the water molecules were located in the Fourier difference map and were restrained to O-H and H•••H distances of 0.85(1) Å and 1.39(2) Å, respectively.Some of the positional parameters of water hydrogen atoms were fixed to their position due to a disorder in the structure or instability of a model.Sulfate ions in 1a•2H 2 O are disordered and were modelled over two positions, with occupancies of exactly 0.5, since they lie on a two-fold axis.One sulfate ion and one valinate residue in 2•4H 2 O are disordered over two positions, and in this case, occupancies were refined to values 0.38:0.62 and 0.58:0.42,respectively.Structures were visualized by MERCURY [40], and the geometrical parameters were calculated by PLATON [41].S1 and S2.
Powder X-ray Diffraction (PXRD).The Panalytical Aeris diffractometer was used to collect powder X-ray diffraction data in a Bragg-Brentano geometry.The source of radiation was CuKα (λ = 1.54056Å).The sample was placed on a Si sample holder and measured.The experiment was conducted at 2θ = 5-40 • with 0.022 • and 15.045 s per step.Data were visualized by the DataViewer program [42].
In vitro cytotoxic activity.Cytotoxicity experiments on compounds 1a•2H 2 O and 2•4H 2 O were performed at the School of Medicine, the University of Zagreb.Experiments were evaluated on two human cell lines: human hepatocellular carcinoma cell lines (HepG2) and human acute monocytic leukemia cancer cell lines (THP-1).The antiproliferative effects of compounds 1a•2H 2 O and 2•4H 2 O were determined by the CellTiter 96 Aqueous One Solution Cell Proliferation Assay (Promega, G3580).This is a tetrazolium-based cell viability assay that measures the metabolic capacity of cells in a culture [43].1a•2H 2 O and 2•4H 2 O were prepared in sterilized water as a stock solution at a concentration of 10 −2 mol dm −3 .Before their application into the bioassay, compounds were diluted in a cell culture medium.Cells were added to plates in an appropriate number per well (50 µL).Plates were incubated overnight at 37 • C in a 5% CO 2 atmosphere.For determining the inhibition of cellular proliferation or the inducement of cytotoxic effects, the CellTiter 96 Aqueous One Solution Cell Proliferation Assay (Promega, G3580) was used.A total of 10 µL of an MTS reagent was dispensed per well.Plates were incubated for 2 h at 37 • C in a 5% CO 2 atmosphere, and the absorbances were recorded at 490 nm using a 96-well Spectramax i3x plate reader.Results were analyzed in the GraphPad Prism software.

Synthetic Comments
Five new ternary coordination compounds containing copper(II) ions; aminoacidate (L-alaninate (L-Ala), L-valinate (L-Val), or L-phenylalaninate (L-Phen)); and 2,2 ′ -bipyridine (bipy) were prepared using a solution-based and/or mechanochemical synthesis, as listed in Figure 1.Two compounds with L-alaninate were synthesized under different conditions-1b•2.5H 2 O was crystallized from an aqueous solution, while 1a•2H 2 O was crystallized from a methanolic solution.In some repeated experiments, 1b•2.5H 2 O was also crystallized from methanolic solution.By using a mechanochemical synthesis, only 1b•2.5H 2 O was prepared.In syntheses with L-valinate, only one compound was formed, 2•4H 2 O, both in aqueous and methanolic solutions, and mechanochemically with a small amount of water or methanol.L-Phenylalaninate produced compounds with a higher fraction of water, 3•8H 2 O and 3•9H 2 O, where the crystallization product depended on a solvent.No crystalline product was formed at a higher water ratio in the solution-based synthesis, while a mixture of water and methanol (1:9 v/v) gave crystals of 3•8H 2 O. From pure methanol, 3•9H 2 O was crystallized from a solution.In some experiments, [Cu(L-Phe) 2 ] n was crystallized from a solution, both from a mixture of water and methanol (1:9 v/v) or pure methanol, with the same number of reactants.3•8H 2 O was synthesized mechanochemically when water was used for liquid-assisted grinding.A scheme of the synthetic procedures is given in Figure 1.The outcomes of solution-based syntheses involving alaninato and phenylalaninato ligands highly depended on unidentified external factors.In these cases, mechanochemical syntheses proved to be very useful in ensuring the reproducibility and purity of the bulk products.

Crystal Structures
The synthesized ternary coordination compounds are composed of complex cations of two different types, with sulfate counterions in 1a•2H 2 O, 1b•2.5H 2 O, and 2•4H 2 O or of complex cations and complex anions in 3•8H 2 O and 3•9H 2 O, all of them being hydrates (Figures S4-S8).The asymmetric unit of 1a•2H 2 O consists of two octahedrally coordinated complex cations (with bridging alaninato ligands), two sulfate anions with an occupancy of 0.5, and two crystallization water molecules (Figure S4).In 1b•2.5H 2 O, the asymmetric unit contains two complex cations, one octahedrally coordinated (with bridging alaninato ligand) and another square-pyramidal, with two symmetrically unique halves of sulfate anions and 2.5 crystallization water molecules (Figure S5).In 1b•2.5H 2 O, sulfate anions and one water molecule lie on a 2-fold axis of rotation.The asymmetric unit of 2•4H 2 O contains one octahedrally coordinated (with bridging valinato ligand) and three squarepyramidal complex cations, two sulfate anions, and four crystallization water molecules (Figure S6).The 3•8H 2 O asymmetric unit contains four complex cations, four complex anions, and 32 crystallization water molecules (Figure S7), and 3•9H 2 O contains two complex cations, two complex anions, and 18 crystallization water molecules (Figure S8).In complex cations, the copper(II) ion is either pentacoordinated by one N,O-donating
The crystal structures of the complex species can be divided into the hydrophobic part, with aliphatic side chains of alanine and valine and aromatic systems of bipyridine and phenylalanine ligands, and the hydrophilic part of aminoacidates, with water molecules and sulfate ions.Due to the specific structure, complex species form predictable supramolecular architectures.Complex cations in  S5 and S6).These chains are connected through coordination  S5 and S6).  1 and S5-Sulfate ions and crystallization water molecules act as hydrogen-bonding bridges betwe 2D layers.In 3•8H2O and 3•9H2O, complex cations and anions, which are in gauche − conf mation, form 1D chains through π interactions (Tables S8 and S9 S7).Complex cations and anions in gauche  S7).
In all the compounds, crystallization water molecules are packed between 2D layers formed by coordination compounds.There is a significant difference in the amount and packing pattern of crystallization water molecules between compounds with aliphatic amino acids and phenylalanine compounds.1a•2H 2 O, 1b•2.5H 2 O, and 2•4H 2 O contain a lower fraction of water molecules (2.0, 5.3, and 4.4% of the unit cell volume, respectively), and the water molecules pack in discrete pockets (Figure 5).On the other hand, 3•8H 2 O and 3•9H 2 O contain higher amounts of water (20.9% and 22.4% of the unit cell volume, respectively), and the water molecules pack into 2D channels (Figure 5).S7).In all the compounds, crystallization water molecules are packed between 2D layers formed by coordination compounds.There is a significant difference in the amount and packing pattern of crystallization water molecules between compounds with aliphatic amino acids and phenylalanine compounds.1a•2H2O, 1b•2.5H2O, and 2•4H2O contain a lower fraction of water molecules (2.0, 5.3, and 4.4% of the unit cell volume, respectively), and the water molecules pack in discrete pockets (Figure 5).On the other hand, 3•8H2O and 3•9H2O contain higher amounts of water (20.9% and 22.4% of the unit cell volume, respectively), and the water molecules pack into 2D channels (Figure 5).

IR (ATR) Analysis
As explained in Section 3.1, we were not able to repeat the synthesis of compounds 1a•2H2O and 3•9H2O to obtain a pure bulk sample for the IR and TG analysis.Infrared and thermogravimetric analyses were made for 1b•2.5H2O,2•4H2O, and 3•8H2O, for which we could reproduce pure samples (Figures S9-S11).As being built by analogous moieties that are almost identically coordinated to the copper(II) centers, the IR spectra of analyzed compounds are similar.Each compound shows one or two broad bands from 3450 cm −1

IR (ATR) Analysis
As explained in Section 3.1, we were not able to repeat the synthesis of compounds 1a•2H 2 O and 3•9H 2 O to obtain a pure bulk sample for the IR and TG analysis.Infrared and thermogravimetric analyses were made for 1b•2.5H 2 O, 2•4H 2 O, and 3•8H 2 O, for which we could reproduce pure samples (Figures S9-S11).As being built by analogous moieties that are almost identically coordinated to the copper(II) centers, the IR spectra of analyzed compounds are similar.Each compound shows one or two broad bands from 3450 cm −1 to 3150 cm −1 that may be assigned to ν(O-H) and ν(N-H) stretching vibrations.These bands indicate extensive hydrogen bonding in crystal structures.In a broad band in the range of 1690-1530 cm −1 , there is an overlap of several vibrational modes of the carboxylate group, the C=N, NH 2 , and OH groups.Absorption bands from 1630 cm −1 to 1600 cm −1 , together with those from 1480 cm −1 to 1390 cm −1 , may be assigned to stretching vibrations of the carboxylate groups ∼ ν asym (COO − ) and ∼ ν sym (COO − ), respectively [21,23,45,46].Absorption bands from 1590 cm −1 to 1490 cm −1 are assigned to ring stretching, C=N stretching (1568 cm −1 ), and ring bending vibrations.Absorption bands at 1218-1203 cm −1 and broad bands between 1150 and 1000 cm −1 correspond to the S=O stretching modes of sulfate ions, and those from 1000 cm −1 to 600 cm −1 belong to ring-H out-of-plane bending, ring in-plane bending, and ring torsion modes.Bands at 557 cm −1 and 548 cm −1 correspond to stretching vibrations that indicate Cu-O bonding, while those close to 460 cm −1 indicate Cu-N bonding.

Thermogravimetric Analysis
The TGA for 1b•2.S10.In general, experimentally determined Cu contents are in good agreement with theoretical values.

Conclusions
We have shown that by using different solvents (methanol or water), different hydrates of ternary coordination compounds with copper, 2,2 ′ -bypiridine and amino acidates (L-alaninate, L-valinate and L-phenylalanine), can be obtained: {  O).Despite these differences, some similarities and predictability in the packing of aliphatic and phenylalaninato complexes are evident.The aromatic parts of complexes stack into rods, while carboxylate groups are bonded to a neighboring complex through hydrogen bonds in all of the prepared compounds.In similar synthetic conditions, L-phenylalaninate gave compounds with a significantly higher fraction of crystallization water molecules compared to the L-alaninato and L-valinato compounds.Like other Casiopeina compounds, 1a•2H 2 O and 2•4H 2 O exhibited significant antiproliferative activity towards human hepatocellular carcinoma cell lines (HepG2) and moderate activity towards human acute monocytic leukemia cancer cell lines (THP-1) compared to staurosporine.Both compounds have similar activity, with 1a•2H 2 O being slightly more active towards THP-1 and 2•4H 2 O towards the HepG2 cell line.

5H 2 O, 2 •
4H 2 O, and 3•8H 2 O was performed in the flow of pure oxygen in the temperature range of 25-800 • C at a rate of 10 K min −1 (Figures S12-S14).Although crystals of all compounds are basically stable in the air at room temperature, water loss in all samples starts approximately at 40 • C, especially in the case of 3•8H 2 O, due to its high water content.All samples exhibit complex degradation, and a loss of water is overlapped by a further decomposition of the compound.Only in the case of 1b•2.5H 2 O is loss of water completed at approximately 180 • C, giving a step of 11.0%, which is in good agreement with the theoretical value (10.2%).For all samples, further degradation of the compound continues up to 400 • C and then abruptly ends (430 • C to 460 • C for 1b•2.5H 2 O; 400 • C to 430 • C for 2•4H 2 O; and 420 • C to 450 • C for 3•8H 2 O), leaving residues that slowly decompose up to 750 • C. The residues in crucibles are identified as CuO.Data on CuO contents and corresponding Cu contents are given in Table Figure S1: Powder diffraction pattern of a sample obtained by the mechanochemical synthesis of 1b•2.5H 2 O (blue) compared to the powder diffraction pattern of 1b•2.5H 2 O simulated from the crystal structure data (red); Figure S2: Powder diffraction pattern of a sample obtained by the mechanochemical synthesis of 2•4H 2 O with methanol (purple) or water (blue) compared to the powder diffraction pattern of 2•4H 2 O simulated from the crystal structure data (red); Figure S3: The powder diffraction pattern of a sample obtained by the mechanochemical synthesis of 3•8H 2 O and aging in the air for 5 min (dark blue), 10 min (blue), and 3 months (light blue) prior to measurements compared to the powder diffraction patterns of 3•8H 2 O (red) and 3•9H 2 O (black) simulated from the crystal structure data; Figure S4: ORTEP plot of the asymmetric unit of 1a•2H 2 O with the atom-labelling scheme.Crystallization water molecules were omitted for clarity.Sulfate atoms are disordered in two positions.Displacement ellipsoids were calculated at the 50% probability level.Symmetry operators:i 1/2−x,−1/2 + y,−z; ii 1/2−x,1/2 + y,−z; iii 1/2−x,1/2 + y,1−z; iv 1/2−x,−1/2 + y,1−z; v x,y,1−z; vi −x,y,−z;Figure S5: ORTEP plot of the asymmetric unit of 1b•2.5H 2 O with the atom-labelling scheme.Crystallization water molecules were omitted for clarity.Displacement ellipsoids were calculated at the 50% probability level.Symmetry operators: i −1/2 + x,3/2−y,1-z; ii 1/2 + x,3/2−y,1−z; Figure S6: (a) ORTEP plot of the asymmetric unit of 2•4H 2 O and (b) the atom-labelling scheme of two complex species.Crystallization water molecules and two other complex cations in (b) were omitted for clarity.One sulfate ion and one side chain of L-valinate is disordered in two positions.Displacement ellipsoids were calculated at the 50% probability level.Symmetry operators: i 1/2−x,1/2 + y,2−z; ii 1/2−x,−1/2 + y,2−z; Figure S7: (a) ORTEP plot of the asymmetric unit of 3•8H 2 O and (b) the atom-labelling scheme of two complex species.Crystallization water molecules and six complex species in (b) were omitted for clarity.Displacement ellipsoids were calculated at the 50% probability level; Figure S8: (a) ORTEP plot of the asymmetric unit of 3•9H 2 O, and (b) the atom-labelling scheme of two complex species.Crystallization water molecules and two complex species in (b) were omitted for clarity.Displacement ellipsoids were calculated at the 50% probability level; Figure S9: IR(ATR) spectrum of 1b•2.5H 2 O; Figure S10: IR(ATR) spectrum of 2•4H 2 O; Figure S11: IR(ATR) spectrum of 3•8H 2 O; Figure S12: TGA curve of 1b•2.5H 2 O; Figure S13: TGA curve of 2•4H 2 O; Figure S14: TGA curve of 3•8H 2 O. CCDCs 2363904-2363908 contain the supplementary crystallographic data for this paper.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/structures (accessed on 14 July 2024) or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk.

Table 1 .
Intramolecular bond lengths of copper(II) coordination sphere in compounds 1a•

3•8H 2 O, and 3•9H 2 O were
shown to be difficult to reproduce and to obtain pure products.On the other hand, mechanochemical syntheses proved to be a simple and reliable technique for obtaining

1b• 2.5H 2 O, 2•4H 2 O, and 3•8H 2 O.
Regarding the crystal structures, there is an influence of amino acid residue on crystal packing, with a clear difference in forming complex species and the packing of aliphatic (1a•2H 2