Pyrrolidine and Its Hydrates in the Solid State - Crystal Growth

Subscriber access provided by Stockholm University Library

Article

Pyrrolidine and its Hydrates in the Solid State Lukasz Dobrzycki, Paulina Taraszewska, Roland Boese, and Micha# K. Cyra#ski Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00527 • Publication Date (Web): 28 Aug 2015 Downloaded from http://pubs.acs.org on August 29, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Crystal Growth & Design is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design

1

2

Pyrrolidine and its Hydrates in the Solid State

3

Lukasz Dobrzycki*, Paulina Taraszewska, Roland Boese, Michał K. Cyrański

4

The Czochralski Laboratory of Advanced Crystal Engineering, Faculty of Chemistry, University of

5

Warsaw, Zwirki i Wigury 101, 02-089 Warsaw, Poland

6

Abstract

7

The crystals of a new ambient pressure, high temperature (HT) polymorphs of neat pyrrolidine hemi-

8

and hexahydrates were obtained using the IR laser assisted in situ crystallization method. They were

9

subsequently investigated by single crystal X-ray diffraction and Raman spectroscopy. The transition

10

from HT form to the known low temperature (LT) polymorph of the amine was monitored by X-ray

11

powder diffraction. Although the investigated amine is an analogue to tetrahydrofurane (THF) which

12

forms a stable clathrate hydrate, both binary pyrrolidine hydrates are dissimilar to the THF hydrate. The

13

hexahydrate of pyrrolidine represents a class of semi-clathrates where “guest” molecules are

14

incorporated in the water framework. While cooling it undergoes a phase transition to the fully ordered

15

phase. At higher temperatures the structure contains disordered pyrrolidine and water molecules. The

16

latter adopts the same type of disorder as typical for H-disordered ice polymorphs (e.g. Ih, Ic ...) and

17

most clathrate hydrates.

18 19

20

*Phone: +0048228220211 ext. 360, Fax: +0048228222892, E-mail: [email protected]

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Pyrrolidine and its Hydrates in the Solid State

2

Lukasz Dobrzycki*, Paulina Taraszewska, Roland Boese, Michał K. Cyrański

3

The Czochralski Laboratory of Advanced Crystal Engineering, Faculty of Chemistry, University of

4

Warsaw, Zwirki i Wigury 101, 02-089 Warsaw, Poland

Page 2 of 25

5

*Phone: +0048228220211 ext. 360, Fax: +0048228222892, E-mail: [email protected]

6

Abstract

7

The crystals of a new ambient pressure, high temperature (HT) polymorphs of neat pyrrolidine hemi-

8

and hexahydrates were obtained using the IR laser assisted in situ crystallization method. They were

9

subsequently investigated by single crystal X-ray diffraction and Raman spectroscopy. The transition

10

from HT form to the known low temperature (LT) polymorph of the amine was monitored by X-ray

11

powder diffraction. Although the investigated amine is an analogue to tetrahydrofurane (THF) which

12

forms a stable clathrate hydrate, both binary pyrrolidine hydrates are dissimilar to the THF hydrate. The

13

hexahydrate of pyrrolidine represents a class of semi-clathrates where “guest” molecules are

14

incorporated in the water framework. While cooling it undergoes a phase transition to the fully ordered

15

phase. At higher temperatures the structure contains disordered pyrrolidine and water molecules. The

16

latter adopts the same type of disorder as typical for H-disordered ice polymorphs (e.g. Ih, Ic ...) and

17

most clathrate hydrates.

18

Introduction

19

Pyrrolidine is a cyclic, secondary amine and at ambient conditions is a liquid with melting point -63 °C.1

20

It is an analogue of tetrahydrofurane (THF) or of tetrahydrothiophene (THT) with the oxygen or sulfur,

21

respectively, replaced by an amino group.2,3 The crystal structure of pyrrolidine is known both at low

22

temperature and ambient pressure4 and at ambient temperature and elevated pressures,5 but none of ACS Paragon Plus Environment

2

Page 3 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

them are similar to the THF and THT structures. In the pyrrolidine solid phases the amine moieties are

2

arranged in columns with the rather weak N-H...N hydrogen bonds (HBs). The neat title compound is

3

widely characterized both in the solid state4,5 and using quantum chemical methods;6 however, no

4

information exists about pyrrolidine hydrates as binary systems. This might be expected because THF is

5

known to crystallize with water to form stable clathrate hydrate crystals.7

6 7

Pyrrolidine, however, was investigated for potential clathrate hydrate formation in the presence of

8

methane.8 Clathrate hydrates and (natural) gas hydrates constitute a group of compounds which form

9

host networks with H2O molecules stabilized by strong hydrogen bonds (HBs), with O...O distances of

10

ca. 2.7 Å. In gas clathrates there are voids or cages large enough to adopt guest molecules.9-13 In these

11

types of crystals, water molecules are surrounded by four H2O molecules with hydrogen atoms

12

disordered with half occupancies, like in hexagonal ice.14 The most prominent example of (natural) gas

13

hydrates has methane in the cages, essentially stabilized by van der Waals interactions. They occur

14

mostly as huge deposits on the continental shelves of the oceans or they coexist with natural gas

15

deposits.15 The fact that methane is a strong greenhouse agent which can be released when harvesting

16

this energy source prevents straightforward exploitation.

17 18

There exist three main types of gas clathrates which differ in cage shape, size and number in the unit

19

cell: sI, sII, sH.15 The tetrahydrofurane clathrate hydrate occurs as type sII with only bigger cages

20

occupied by disordered THF molecules whereas the smaller dodecahedron-shaped cavities are left

21

empty. In spite of the fact that there is some space left in the lattice of THF clathrates, the empty host

22

networks are surprisingly stable with melting temperatures around +4 °C.

23

In comparison to THF, pyrrolidine has a similar shape but can act as a HB donor and acceptor; whereas

24

the THF molecule can only offer its lone pair electrons as proton acceptors, which fundamentally alters

25

the situation. For these reasons, it seems more likely that pyrrolidine will form simple hydrates rather

26

than binary clathrates. Such hydrates – if enough water molecules are present – might form semiACS Paragon Plus Environment

3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 25

1

clathrates16 where the amine moieties are incorporated into a water framework. Thus in our work we

2

decided to investigate the ability of pyrrolidine to form hydrates.

3

Experimental Section

4

X-ray powder diffraction experiments on flash-cooled liquid amine sealed in thin-wall glass capillaries

5

with a diameter of about 0.8 mm were performed on a Bruker D8 Discover diffractometer17 equipped

6

with capillary stage, sealed tube with Cu anode, Goebel focusing mirror and LT device. Further analyses

7

of the obtained data, such as generation of “waterfall” and “density” plots were performed in the

8

Mathematica package.18 Because of a severe texture effect in the obtained pyrrolidine needle-shaped

9

powder, the determination of the unit cell parameters was not successful.

10 11

Samples adequate for single crystal diffraction experiments were obtained with the help of IR laser

12

assisted in situ crystallization techniques.19 Thin-wall glass capillaries containing the sealed liquid of the

13

desired amine or THF concentration were fixed in the cold gas stream of the LT device on the

14

goniometer of a Bruker D8 Venture diffractometer equipped with a CMOS area detector. Detailed

15

crystallographic data together with refinement parameters for all structures obtained are collected in

16

Table 1, whereas thermal ellipsoid plots including the numbering scheme of atoms are displayed in

17

Figure 1. The diffraction data were measured using the φ-scan method. In the case of some experiments,

18

this resulted in unsatisfactory (i.e. less than 98%) completeness of the data because of preferred

19

orientation of the crystals grown in the capillary and the limits of the goniometer movements during the

20

data collection. In all cases, except for the THF clathrate hydrate, the obtained samples were

21

oligocrystalline and/or twinned.

22 23

The data were integrated and scaled using the Bruker suite of programs.20 All the structures were solved

24

by direct methods and refined using SHELXL.21 The atomic scattering factors were taken from the

25

International Tables.22 The structures of twinned crystals were refined based on two components

26

(instruction HKLF 5). After final convergence, the reflection data were transformed to the regular ACS Paragon Plus Environment

4

Page 5 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

HKLF 4 format for final refinements.23 During the refinement of the pure amine, the structures of all

2

non-H atoms were refined anisotropically. The methylene hydrogen atoms were placed in calculated

3

positions and refined using the riding model and with isotropic thermal parameters set 1.2 times greater

4

than the corresponding C atoms. The hydrogen atom of the amino group was free to refine, including

5

position and isotropic thermal parameter. In the structure of the hemihydrate all heavy atoms were

6

treated anisotropically. The methylene hydrogen atoms were placed at calculated positions and refined

7

using a riding model and with isotropic thermal parameters set 1.2 times greater than the corresponding

8

C atoms. Hydrogen atoms engaged in HBs were refined with distance restraints (DFIX 0.95 0.03

9

instruction) while their isotropic thermal parameters were set to 1.5 times greater than Ueq of the

10

corresponding heavy atoms. A similar procedure was applied for the LT phases of pyrrolidine

11

hexahydrate, measured at -173 °C, -123 °C and -83 °C. In the structures of the HT form of hexahydrate

12

a number of geometric constraints were applied to preserve a reasonable geometry for the pyrrolidine

13

molecule. The hydrogen atoms of the water molecules were positioned on the lines linking neighboring

14

O atoms with O-H distances set to 1 Å. The occupancies of the water H atoms were fixed at 50% of the

15

occupancies of the O atom divided by the multiplicities of the positions of the H atoms. The thermal

16

parameters of the H atoms were fixed at the level of 1.2 or 1.5 times greater than Ueq for the

17

corresponding heavy atom. In the structure of THF the hydrogen atoms were placed on lines between

18

the nearest O atoms with O-H distances set to 1 Å and isotropic thermal parameters set at 1.5 times

19

those of the corresponding O atoms. Hydrogen atoms of THF molecule were not allocated due to severe

20

disorder. The crystal structures were deposited in the Cambridge Crystallographic Data Centre with the

21

following numbers: CCDC 1059233-1059243.

22 23

Raman spectra for the obtained pure crystalline phases were acquired using a confocal probe connected

24

to an InduRAM system, with the help of the 532nm Nd laser excitation line. The DSC measurements for

25

the neat amine and its water solution, containing one mole of the amine per 6 moles of H2O sealed in an

26

Al crucible, were performed on a Netzsch DSC 204 F1 Phoenix system. The measurement was based on ACS Paragon Plus Environment

5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 25

1

two and three cooling-heating cycles which were averaged thereafter for the pure amine and its mixture

2

with water, respectively. The heating/cooling rate was 10 K min-1 and the protective gas used was of 5N

3

purity N2.

4 5

Results and Discussion

6

In our preliminary experiments, we focused on the pure pyrrolidine. Because crystallographic studies

7

using single crystals were already known, we decided to analyze the phase behavior of pyrrolidine with

8

a powder diffractometer using flash crystallization. The liquid amine was sealed in a thin-walled glass

9

capillary and was rapidly cooled down from RT to -83 ºC and kept constant. Multiple 2θ scans repeated

10

approximately every hour at -83 ºC revealed gradual changes of the powder pattern. Powder diagrams

11

(2θmax = 18°) for the neat amine are presented in Figure 2a whereas the “density plot” (simulated

12

Guinier-Simon experiment24) showing changes between each scan is presented in Figure 2b. Such

13

changes of the powder diagrams indicate that there are two ambient pressure transformation polymorphs

14

of pyrrolidine. The non-continuity of the lines indicates a first order phase transition. Unfortunately we

15

were unable to index the two powder patterns belonging to different phases of pyrrolidine although the

16

unit cell parameters of one phase were known from single crystal data. This is probably due to the

17

strong preferred orientation effect.

18 19

Thus, a reasonable way to investigate pyrrolidine in the solid state and the search for another polymorph

20

seems to be the use of IR laser assisted in situ crystallization technique19 with a single crystal

21

diffractometer. The crystallization process was performed at -66 °C, which is close to the melting point

22

of the amine. The outcome of the experiment was an oligocrystalline sample.11 We found that the

23

obtained phase corresponds to a new orthorhombic, high temperature polymorph of pyrrolidine.

24

Moreover, the crystals belonging to the Pbca space group undergo an enantiotropic phase transition25 to

25

a monoclinic α form. There is no damage observed during this reversible process. However, because of

26

the symmetry change of the lattice at low temperature, the crystals got twinned. The temperature ACS Paragon Plus Environment

6

Page 7 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

dependent transformation between the HT and the LT phase shows a distinct hysteresis. So the HT form

2

is stable at -108 °C whereas the LT form can exist even at -68 °C after overheating – compare single

3

crystal X-ray structures established at -66 °C, -108 °C and -138 °C, -68 °C for HT and LT polymorphs

4

respectively. Such behavior is in disagreement with observation from the powder diagrams where even

5

at -83 °C the HT form is transformed to the LT form within tens of minutes. This can be explained

6

based on the size of the crystal grains obtained, leading to fine powder during flash cooling; millimeter-

7

sized needle crystals were only achieved when moving the zone melting along the capillary.

8 9

Calorimetric measurements should illuminate the problem of the phase transition in pyrrolidine crystals.

10

The DSC curves for cooling and heating of pyrrolidine are presented in Figure 3. The measurement

11

shows no signal indicating a HT→LT phase transition on cooling but only strong peak associated with

12

formation of a solid phase from the melt at -85.1 °C. However, in the reverse process there is a small

13

signal at -81.2 °C before the melting starts at -62.8 °C. This weak signal represents the LT→HT phase

14

transition of the solid pyrrolidine. So in a cooling process with a speed of 10K/min the LT phase of the

15

pyrrolidine crystallizes at once. We performed an experiment at a speed of 2K/min with similar results

16

and found only one signal present on cooling. It should be explained that the crystallization of HT

17

polymorph of the pyrrolidine had to be performed in rather slow way. The reasonable method to obtain

18

this phase is an IR laser based in situ crystallization technique. At temperatures just below the melting

19

point the slow movement of the laser beam along the capillary with sealed solid amine allowed the

20

crystallization of desired HT phase. According to the DSC measurement, the amine crystallizes at -

21

85.1 °C, whereas the PWD flash crystallization was performed at -83 °C. This successful crystallization

22

of the HT polymorph on the PWD diffractometer was probably due to the temperature gradient along

23

the cooled capillary containing the amine. During the repeated X-Ray scans on the PWD diffractometer

24

the freshly crystallized HT polymorph was slowly converted to the LT one.

25


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 25

1

The crystal structure of the HT polymorph of pyrrolidine is in general very similar to the LT and the

2

high pressure forms. In the crystal lattice the amine molecules forming N-H...N hydrogen bonds are

3

arranged in parallel columns. The NH(axial)-off envelope conformation5 in the HT form is the same as

4

in the LT form. The comparison of the packing of ambient pressure pyrrolidine polymorphs is presented

5

in Figure 4. The LT↔HT phase transition is associated with a reorientation of the columns of

6

molecules. There is also a distinct change in the geometry of the HBs between molecules which is

7

specified in Table 2 but the 1D topology of these interactions is preserved in both HT and LT

8

polymorphs. In the HT polymorph, N...N distances are shorter in spite of the higher temperature, so this

9

effect has to be associated with changes of orientation of molecules in the columns during the phase

10

transition.

11 12

The phase transitions during heating up from -173 °C to above the melting point of the pure pyrrolidine

13

were also investigated by Raman spectroscopy of the same sample which the single crystal X-ray

14

experiments were performed on. The collection of Raman spectra in the region of C-H and N-H

15

stretching modes is presented in Figure 5. The spectra acquired during cooling of the HT polymorph

16

down to -136 °C and heating it up above the melting point illustrate reversibility of the phase transition.

17

On cooling from -66 °C the relatively weak, smeared and complex peak observed for N-H stretching is

18

shifted form 3180cm-1 to lower frequencies. Between -118 °C and -128 °C there is a distinct change of

19

the spectrum profile associated with the HT->LT phase transition. During heating up, the bands became

20

wider and weaker. After the LT → HT phase transition in the solid state around T = -66 °C there is

21

again an evident and discrete change in the spectrum. Moreover, the N-H stretching mode close to

22

3200cm-1 in the HT phase is shifted back again to 3180 cm-1, which indicates a strengthening of the

23

hydrogen bond in comparison to the LT phase. In the liquid phase, further broadening of bands

24

associated with an increase of the N-H bond strength is observed. This is associated with a large number

25

of interactions existing in such a dynamic system. There is, however, one intriguing observation based

26

on the Raman spectra. As a secondary amine, pyrrolidine should have only one N-H stretching band ACS Paragon Plus Environment

8

Page 9 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

whereas in the solid state two bands are observed. This is clearly visible at lower temperatures but also

2

for the HT phase: the N-H band is a combined, slightly smeared peak. Such observations coming from

3

Raman spectra may indicate that the behavior of the molecules in the pyrrolidine crystal lattice is more

4

complex, probably due to Davydov splitting.26

5 6

Referring to the original question - whether pyrrolidine is able to form clathrates – our initial attempts to

7

crystallize the amine used relatively small amounts of water. Because pyrrolidine can serve

8

simultaneously as a HB donor and acceptor it is suggested to start the crystallization with the molar ratio

9

1:0.5 pyrrolidine:water, which should lead to a hemihydrate. The mixture was flash cooled in a capillary

10

and measured on a powder diffractometer. Only a glassy phase was observed as illustrated in Figure 6,

11

so we applied the IR laser assisted in situ crystallization technique on a single crystal diffractometer,

12

where the crystallization turned out to be more challenging than for the neat pyrrolidine. Due to

13

vitrification of the sample in the capillary, high energy IR laser pulses were needed to start the

14

crystallization process at about -173 °C. When a crystalline mass was visible, the temperature was

15

elevated to -83 °C with subsequent movement of the molten zone, generated by the IR laser which again

16

resulted in oligocrystalline material, but of sufficient quality to collect single crystal diffraction data. As

17

expected, the resulting structure was the hemihydrate of pyrrolidine. The unit cell of this compound

18

contains two groups of amine-water-amine [see Figure 1 e)]. Within one of the groups, all pyrrolidine

19

molecules have the same conformation of NH(axial)-off envelope whereas the other group contains a

20

one-to-one mixture of NH(axial)-off and α-CH2-off H-(axial) amine conformers, this latter one having

21

conformational chirality. Due to the centrosymmetric space group, both conformers coexist resulting in

22

a racemic mixture.3 One of the NH(axial)-off envelope pyrrolidine conformers is disordered in its

23

hydrophobic part which is manifested by elongated ADPs of the corresponding β-carbon atoms. In the

24

crystal structure, the strongest hydrogen bonds are formed between water and the pyrrolidine acting as

25

HB donors. In this case, N...O distances range from 2.85 to 2.92 Å. There are also weaker HBs between

26

molecules involving amine hydrogen atoms. These contacts distances are in the range 3.11 – 3.19 Å. ACS Paragon Plus Environment

9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 25

1

Due to these interactions, the molecules are grouped in layers perpendicular to (100) which is illustrated

2

in Figure 7. Such weak interactions are responsible for the relatively low stability of the crystals – the

3

melting point is about -72 °C – which is lower than for the pure amine.

4 5

With a higher amount of water we observed the formation of only one crystalline phase – the

6

hexahydrate of pyrrolidine. The crystals were also obtainable from other mixtures with

7

pyrrolidine:water molar ratios from 1:3 up to 1:10. At -4 °C, using in situ crystallization, we obtained

8

oligocrystalline samples sufficient for single crystal X-ray experiments. The pyrrolidine hexahydrate

9

crystals are severely disordered at -4 °C. The closest environment of the amine molecule in the crystal

10

lattice is presented in Figure 1 f) in the form of a thermal ellipsoid plot. The pyrrolidine is positioned in

11

mm2 site symmetry which results in symmetry-imposed disorder over two sites. The nitrogen atom of

12

the amino group shares the same position for both sites; however, it’s large ADPs indicate a relatively

13

high degree of disorder. All hydrogen atoms of the H2O molecules are equally disordered over two sites

14

which is typical in H-disordered ice structures.27 The water molecules form a 3-D network via relatively

15

strong hydrogen bonds (average O...O distance 2.808 Å). The amine molecules are incorporated into the

16

water network by strong N-H...O HBs. The disordered hydrophobic part of pyrrolidine is located in

17

cavities of the 3-D water network. In this respect, the hexahydrate of pyrrolidine can be treated as semi-

18

clathrate16 where amine together with water molecules act as building blocks for the whole structure. In

19

the structure of the hexahydrate measured at -4 °C positional disorder of the water O1 atom can be

20

observed [see Figure 1 f)]. The highest occupancy for this atom is 95%; the residual 5% of electron

21

density is attributed to the alternative position of this atom, labeled O1B. This effect is probably

22

associated with the onset of the crystal structure disintegration on further heating and the following

23

melting process at a temperature just above -4 °C. At lower temperatures, e.g. -63 ºC, no such disorder

24

of the water framework is observed – see Figure 1 g). Further cooling of the hexahydrate crystals lead to

25

a solid-state phase transition accompanied by lowering of the symmetry (SG: Cmcm → P21/m). This

26

phase transition is associated with twinning. The resulting monoclinic phase is fully ordered including ACS Paragon Plus Environment

10

Page 11 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

both pyrrolidine and water molecules – see Figure 1 h). The structure of the pyrrolidine hexahydrate

2

was determined at five temperatures: -4 ºC, -63 ºC, -83 ºC, -123 ºC and -173 ºC (see Table 1) and a

3

comparison of changes in crystal lattice of the hexahydrate is presented in Figure 8. From the structure

4

of the LT ordered polymorph of the hexahydrate it is evident that the crystals contain pyrrolidine in its

5

NH(equatorial)-off envelope conformation, which is expected to be energetically preferred compared to

6

the NH(axial)-off envelope option.28,29 This probably results from more freedom of the amine located in

7

the voids formed by the water framework. During this phase transition, the crystal architecture with

8

amine molecules located in water-based channels is preserved in general. The walls of the channels

9

hosting pyrrolidine are based on 4- and 6-membered rings created by interacting water molecules. The

10

ordering of hydrogen atoms of water molecules in the hexahydrate LT phase is somehow similar to the

11

behavior of some phases of ice. The XI polymorph of ice can be obtained by cooling a diluted KOH

12

aqueous solution below -201 °C.30 Ice XI is in fact the ordered form of hexagonal ice, Ih. By analogy,

13

pyrrolidine is expected to act as a “dopant” helping to create a water structure with such an architecture

14

which can only order at certain temperatures.

15 16 17

Raman spectroscopy is an ideal tool for following the dynamics of atoms in crystals undergoing phase

18

transitions. The collection of the Raman spectra in the range 2600 cm-1– 3600 cm-1 recorded for the

19

hexahydrate crystals during heating to 17 °C is presented in Figure 9. The spectrum at -173°C has very

20

sharp and strong bands, corresponding mainly to O-H and N-H stretching modes. The signal at the

21

higher wavelength corresponds to the amine N-H vibration. The relatively high intensity of O-H

22

complex bands confirms the H-order of water molecules. At higher temperatures, the dynamics of the

23

molecules increases, which is associated with a decrease of intensity of the signals. During the phase

24

transitions, sudden changes of the spectrum are observed, leading to a smearing out of the signals. In the

25

liquid phase, (5.22 molar solution of pyrrolidine in H2O) the characteristic broad signal for water is seen

26

together with a distinct doublet for N-H stretching which originates from two different conformations of ACS Paragon Plus Environment

11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 25

1

the amino group. A rough estimation for the melting temperature of the pyrrolidine hexahydrate is ca.

2

-4 °C.

3 4

It was possible to determine thermal stability of the system using differential scanning calorimetry. Thus

5

a proper pyrrolidine solution containing 6 mole of water per 1 mole of the amine was prepared.

6

Resulting DSC curves are presented in Figure 10. The hexahydrate phase starts to crystallize close to

7

-19 °C and the crystals are stable up to -3.6 °C. Indeed, the crystal structure of this compound

8

determined at -4 °C is really at the edge of its stability. For the solid-state reversible phase transition, a

9

small hysteresis at about 6 °C can be observed.

10 11

In spite of comparable shape, pyrrolidine does not form structurally similar hydrates like THF. In

12

pyrrolidine hexahydrate there are strong HBs present between the amine molecules and the water

13

network. The question arises if there is any evidence of guest-water HBs in the THF hydrate. This

14

cannot be evaluated based on the structural data of the THF clathrate6 retrieved from the CSD31 due to

15

the lack of atomic coordinates. These data are based on a powder diffraction X-ray experiment. To

16

answer this question we crystallized an sII-type clathrate hydrate of THF using in situ IR laser assisted

17

methods and carefully analyzed the structural data based on the single crystal obtained. The closest

18

environment of the THF molecule and the atomic displacement parameters are presented in Figure 1 i).

19

The final data and refinement parameters for the first single crystal structure determination of THF

20

hydrate clathrate are also presented in Table 1. The disordered THF molecules occupy the bigger 51264

21

cages while the 512 cavities are left empty. In the crystal lattice there is no evidence of classical HB32

22

formation between THF and water molecules. Moreover, the water framework in this system is not

23

disordered in terms of alternative positions of O atoms. The O atoms of the disordered THF species are

24

pointing to the centers of 6-membered rings constituting the polyhedra formed by water molecules with

25

distances Owater-OTHF around 3.70 to 3.77 Å.

26 ACS Paragon Plus Environment

12

Page 13 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Conclusions

2

The known ambient pressure α-form of pyrrolidine4 undergoes an enantiotropic phase transition just

3

below its melting temperature into the hitherto unknown high temperature orthorhombic polymorph.

4

Both phases are ordered and structurally closely related, preserving the same NH(axial)-off envelope

5

conformation of this highly flexible molecule. The packing of all known forms of pyrrolidine is

6

different to tetrahydrofuran (THF) and tetrahydrothiophene (THT)2,3 due to the lack of heteroatom-

7

bound hydrogen in these two analogues. Pyrrolidine forms binary systems with water; however, they are

8

of a different type than the THF hydrate clathrate type sII, for which the first single-crystal structure is

9

presented here (see Table 1). There are no literature references to diffraction data for the THT

10

hydrate(s). As expected, one of the hydrates of pyrrolidine is a hemihydrate: in the structure two amine

11

species are coupled by one water molecule. In general (one exception) the conformation of the amine is

12

the same as in the crystals of the neat compound i.e. NH(axial)-off envelope. This is not the case for the

13

hexahydrate of pyrrolidine. In this system, the amine attains the shape of NH(equatorial)-off envelope

14

which is the more energetically favorable conformation.28 Pyrrolidine anchored by N-H HBs in the

15

water framework has probably more conformational freedom for the aliphatic site of the molecule

16

located in niches formed by H2O species. At temperatures close to the melting point, the water

17

framework starts disordering, including the position of O atoms, which is a prelude to the disintegration

18

of the complete structure and transformation to a liquid.

19 20

The hexahydrate crystals undergo an enantiotropic phase transition to its fully ordered form, including

21

ordered hydrogen atoms, resulting in a homodromic system.33-35 This compares to ice XI as a proton

22

ordered form of ice Ih and possible equilibria between these two phases of ice.27,30 We were not able to

23

find more hydrates of pyrrolidine. Nevertheless, this complex system exists with methane7 where the

24

hydrocarbon can act as a templating agent at elevated pressures for the formation of gas clathrate

25

hydrates.

26 ACS Paragon Plus Environment

13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 25

1

References

2

(1)

Bradley, J.-C.; Williams, A.; Lang, A., "Jean-Claude Bradley Open Melting Point Dataset", 2014

3

(2)

Luger, P.; Buschmann, J., Angew. Chem., Int. Ed., 1983, 22, 410.

4

(3)

Boese, A. D.; Boese, R., Cryst. Growth Des., 2015, 15, 1073–1081.

5

(4)

Bond, A. D.; Davies, J. E.; Parsons, S., Acta Cryst. C., 2008, 64, o543.

6

(5)

Dziubek, K. F.; Katrusiak, A., Phys. Chem. Chem. Phys., 2011, 13, 15428.

7

(6)

Carballeira, L.; Pe´rez-Juste, I.; Van Alsenoy, C., J. Phys. Chem. A, 2002, 106, 3873.

8

(7)

Sargent, D. F.; Calvert, L. D., J. Phys. Chem., 1966, 70, 2689-2691.

9

(8)

Shin, W.; Park, S.; Ro, H.; Koh, D.-Y.; Seol, J.; Lee, H., J. Chem. Thermodynamics, 2012, 44, 20-

10

25.

11

(9)

12

(10) Sloan E. D., Nature, 2003, 426, 353-359.

13

(11) Kirchner, M. T.; Boese, R.; Billups, W. E.; Norman, L. R., J. Am. Chem. Soc. 2004, 126, 9407.

14

(12) Udachin, K. A.; Ratcliffe, C. I.; Ripmeester, J. A., Angew. Chem. Int. Ed. 2001, 40, 1303.

15

(13) Udachin, K. A.; Ratcliffe, C. I.; Ripmeester, J. A., Angew. Chem. 2001, 113, 1343.

16

(14) Kuhs, W. F.; Lehmann, M. S., “The structure of ice-Ih”, Water Science Reviews 2 (Cambridge

17 18 19

Davy, H. P., Trans. R. Soc. 1811, 101, 155.

University Press: 1986) 1-66. (15) Sloan, E. D.; Koh, C. A., “Clathrate Hydrates of Natural Gases”, 3rd Edition, Taylor & Francis/CRC Press, Boca Raton, FL, USA, 2008.

20

(16) Fowler, D. L.; Loebenstein, W. V.; Pall, D. B.; Kraus, C. A., J. Am. Chem. Soc., 1940, 62, 1140.

21

(17) DIFFRAC EVA, Bruker AXS, Madison, Wisconsin, USA, 2012.

22

(18) Wolfram Research, Inc., Mathematica, Version 7.0, Champaign, IL, 2008.

23

(19) Boese, R., Z. Kristallogr. 2014, 229, 595.

24

(20) SAINT,. Bruker AXS Inc., Madison, Wisconsin, USA, 2013; SADABS,. Bruker AXS Inc.,

25

Madison, Wisconsin, USA, 2012; TWINABS,. Bruker AXS Inc., Madison, Wisconsin, USA,

26

2012. ACS Paragon Plus Environment

14

Page 15 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

(21) Sheldrick, G. M., Acta Crystallogr., 2008, A64, 112–122.

2

(22) International Tables for Crystallography, Ed. Wilson A. J. C., Kluwer: Dordrecht, 1992, Vol. C.

3

(23) Farrugia, L. J., J. Appl. Cryst. 2012, 45, 849-854.

4

(24) Simon, A., J. Appl. Cryst., 1971, 4, 138.

5

(25) Bernstein J., “Polymorphism in Molecular Crystals”, Clarendon Press, Oxford, May, 2002.

6

(26) Davydov A. S., Zh. Eksp. Teor. Fiz., 1948, 18, 210.

7

(27) Chaplin M. F., web page: http://www1.lsbu.ac.uk/water/water_structure_science.html

8

(28) Caminati, W; Dell’ Erba, A.; Maccaferri, G; Favero, P. G., J. Mol. Spectrosc., 1998, 191, 45.

9

(29) Kunitski, M.; Riehn, C.; Matylitsky, V. V.; Tarakeshwar, P.; Brutschy, B., Phys. Chem. Chem.

10

Phys., 2010, 12, 72.

11

(30) Fukazawa, H.; Ikeda, S.; Mae, S., Chem. Phys. Lett., 1998, 282, 215-218.

12

(31) Allen, F. H., Acta Crystallogr. 2002, B58, 380. Version 1.17, 2014.

13

(32) Desiraju, G. R.; Steiner, T., “The Weak Hydrogen Bond: In Structural Chemistry and Biology”,

14

IUCr Monographs on Crystallography, Vol. 9, Oxford: Oxford University Press/International

15

Union of crystallography, 1999.

16 17

(33) Saenger W., Inclusion Compounds, Vol. 2, edited by J. L. Atwood, J. E. D. Davies & D. D. MacNicol, ch. 8, p. 253. London: Academic Press. 1984;

18

(34) Mootz D.; Schilling M., J. Am. Chem. Soc., 1992, 114, 7435–7439.

19

(35) Merz, K.; Kupka, A., Cryst. Growth Des., 2015, 15, 1553–1558.

20 21 22

Acknowledgements

23

The X-ray measurements were performed in the Czochralski Laboratory of Advanced Crystal

24

Engineering (Faculty of Chemistry, University of Warsaw) established by generous support from the

25

Polish Ministry of Science and Higher Education (grant No. 614/FNiTP/115/2011). LD greatly

26

acknowledges the Foundation for Polish Science for the Homing Plus/2011-4/5 grant based on the ACS Paragon Plus Environment

15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 25

1

European Union Regional Development Fund. MKC acknowledges National Science Center (grant

2

NCN 2011/03/B/ST4/02591). All authors are grateful to dr Siân Howard for proof-reading the

3

manuscript.


16

Page 17 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 1. Crystal data and structure refinement for described compounds.

Composition M T/ º C λ/ Å Space group unit cell parameters/ Å, ° V/ Å3 Z, Dx/ g·cm-3 µ/ mm-1 F(000) θmin, θmax Reflections collected/ independent Completness Tmax, Tmin Reflections/ constrains/ parameters GooF on F2 R [I>2σ(I)] R (all data) ρmax, ρmin/ e·Å-3

Pyrrolidine LT phase

Pyrrolidine LT phase

Pyrrolidine HT phase

Pyrrolidine HT phase

Hemihydrate

Hexahydrate phase LT



Hexahydrate phase HT

Hexahydrate phase HT

THF clathrate

C4H9N 71.12 -138.1(1) 0.71073 P21/c a=8.6611(17)Å b=5.2080(9)Å c=10.672(2)Å β=110.561(6)° 450.73(15) 4, 1.048 0.063 160 2.51°, 25.04°

C4H9N 71.12 -68.0(1) 0.71073 P21/c a=8.740(2)Å b=5.2169(13)Å c=10.816(3)Å β=110.284(8)° 462.6(2) 4, 1.021 0.062 160 4.02°, 25.03°

C4H9N 71.12 -108.2(1) 0.71073 Pbca a=5.3132(6)Å b=10.6916(15)Å c=16.268(2)Å

C4H9N 71.12 -66.2(1) 0.71073 Pbca a=5.3112(5)Å b=10.7769(13)Å c=16.432(2)Å

C4H9N+6×H2O 179.22 -123.1(1) 0.71073 P21/m a=7.7773(12)Å b=8.1088(12)Å c=8.6962(13)Å β=103.605(4)° 533.03(14) 2, 1.117 0.103 200 2.41°, 25.05°

C4H9N+6×H2O 179.22 -83.2(1) 0.71073 P21/m a=7.8605(9)Å b=8.1447(8)Å c=8.6837(10)Å β=103.559(4)° 540.45(10) 2, 1.101 0.101 200 2.41°, 25.04°

C4H9N+6×H2O 179.22 -4.2(1) 0.71073 Cmcm a=13.044(2)Å b=10.3955(16)Å c=8.2250(11)Å

C4H8O+17×H2O 378.38 -53.2(1) 0.71073 Fd-3m a=17.217(1)Å

940.5(2) 8, 1.005 0.061 320 3.78°, 25.05°

C4H9N+6×H2O 179.22 -173.2(1) 0.71073 P21/m a=7.7347(7)Å b=8.0940(8)Å c=8.6976(8)Å β=103.683(3)° 529.06(9) 2, 1.125 0.103 200 2.41°, 25.05°

C4H9N+6×H2O 179.22 -63.2(1) 0.71073 Cmcm a=12.9839(19)Å b=10.3156(15)Å c=8.2093(10)Å

924.1(2) 8, 1.022 0.062 320 2.50°, 25.05°

2 × C4H9N+H2O 160.2 -83.2(1) 0.71073 P21/c a=19.2938(10)Å b=12.3590(6)Å c=8.6638(4)Å β=98.0980(10)° 2045.30(17) 8, 1.041 0.069 720 2.89°, 25.05°

1099.5(3) 4, 1.083 0.100 400 2.52°, 25.05°

1115.3(3) 4, 1.067 0.098 400 2.51°, 25.02°

5103.2(6) 8, 0.985 0.103 1680 3.93°, 27.96°

786 / 786 [Rint=0*]

787 / 787 [Rint=0*]

10086 / 812 [Rint=0.0347]

4206 / 833 [Rint=0.0328]

24068 / 3326 [Rint=0.0480]

932 / 932 [Rint=0*]

963 / 963 [Rint=0*]

946 / 946 [Rint=0*]

6208 / 487 [Rint=0.0332]

6199 / 496 [Rint=0.0275]

4130 / 330 [Rint=0.0221]

98.4% 0.981, 0.981

96.1% 0.982, 0.982

99.0% 0.982, 0.982

99.3% 0.982, 0.982

91.8% 0.980, 0.980

92.0% 0.970, 0.970

94.1% 0.9670, 0.970

90.9% 0.970, 0.970

89.4% 0.971, 0.971

89.7% 0.971, 0.971

98.5% 0.970, 0.970

786 / 0 / 50

787 / 0 / 50

812 / 0 / 50

833 / 0 / 50

3326 / 8 / 223

932 / 9 / 80

963 / 9 / 80

946 / 9 / 80

487 / 5 / 42

496 / 5 / 43

330 / 0 / 21

1.189 R1=0.0601, wR2=0.1721 R1=0.0673, wR2=0.1769 0.225, -0.212

1.056 R1=0.0530, wR2=0.1417 R1=0.0611, wR2=0.1498 0.176, -0.129

1.097 R1=0.0502, wR2=0.1356 R1=0.0556, wR2=0.1405 0.189, -0.101

1.085 R1=0.0551, wR2=0.1403 R1=0.0701, wR2=0.1532 0.197, -0.146

1.073 R1=0.0642, wR2=0.1659 R1=0.0879, wR2=0.1896 0.341, -0.22

1.146 R1=0.0488, wR2=0.1440 R1=0.0553, wR2=0.1525 0.312, -0.275

1.186 R1=0.0434, wR2=0.1149 R1=0.0472, wR2=0.1184 0.225, -0.262

1.104 R1=0.0556, wR2=0.1543 R1=0.0627, wR2=0.1641 0.360, -0.207

1.124 R1=0.0545, wR2=0.1400 R1=0.0574, wR2=0.1462 0.308, -0.188

1.161 R1=0.0645, wR2=0.1613 R1=0.0660, wR2=0.1636 0.334, -0.176

1.127 R1=0.0431, wR2=0.1263 R1=0.0493, wR2=0.1307 0.172, -0.299

* Crystals twinned by pseudomerohedry. Data reduction and scaling based on two twin components. Structure refinement with option HKLF5 and BASF parameter free to vary. Final refinement with use of HKLF4 instruction for converted data.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 25

Table 2. Comparison of HB geometry in the HT and the LT phase of pyrrolidine polymorphs. T [ºC] -66 -108 -138 -68

Polymorph HT HT – supercooling LT (α) LT (α) – overheating

N...N distance

N-H...N angle

3.158(2) Å 3.137(1) Å 3.164(2) Å 3.190(2) Å

166(2)° 166(2)° 167(3)° 167(2)°


18

Page 19 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Thermal ellipsoid plots at 50% probability level and numbering scheme for all obtained


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

structures: a) – d) polymorphs of pyrrolidine, e) pyrrolidine hemihydrate, f) – h) polymorphs of pyrrolidine hexahydrate, i) THF clathrate hydrate.

Figure 2. Powder diagrams collected for the flash-cooled pyrrolidine down to -83 °C recorded every hour presented in form of “waterfall plot” a) and “density plot” b).

Figure 3. DSC diagram recorded for the pyrrolidine.


20

Page 21 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Packing diagrams of the LT and HT polymorphs of pyrrolidine.

Figure 5. Raman spectra of crystalline and liquid pyrrolidine recorded at different temperatures.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

Figure 6. Powder diagram recorded for a flash-cooled mixture at -173 °C containing pyrrolidine and water in a 1:0.5 molar ratio.

Figure 7. Crystal packing of pyrrolidine hemihydrate. Hydrogen atoms omitted for clarity.


22

Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Comparison of LT and HT phases of pyrrolidine hexahydrate. Hydrogen atoms omitted for clarity.

Figure 9. Raman spectra of crystalline hexahydrate of pyrrolidine and mother liquid recorded at different temperatures.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 25

Figure 10. DSC diagram recorded for the mixture of pyrrolidine and water in molar ratio 1:6.


24

Page 25 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Use Only

Pyrrolidine and its Hydrates in the Solid State Lukasz Dobrzycki, Paulina Taraszewska, Roland Boese, Michał K. Cyrański

Synopsis An ambient pressure polymorph of pyrrolidine is presented together with two hemi- and hexahydrate structures of this amine. The latter hydrate undergoes an enantiotropic phase transition to a fully-ordered phase which allows determination of hydrogen atoms positions.


25

Pyrrolidine and Its Hydrates in the Solid State - Crystal Growth

Recommend Documents