Vibrational and thermodynamic properties and molecular motions in the incommensurate crystal of morpholinium tetrafluoroborate studied by 1H NMR

Article abstract of DOI:10.1016/j.chemphys.2011.01.002

Dynamics of molecules in morpholinium tetrafluoroborate, [NH ₂(C₂H₄)₂O][BF₄], has been studied in a wide temperature range by means of AC calorimetry, ¹H NMR and infrared spectroscopy. Simultaneously, DFT (density functional theory) calculations of the molecular structure and normal vibration frequencies have been performed by using various functionals for initial structural data of the low temperature phase. Infrared spectra of polycrystalline [NH ₂(C₂H₄)₂O][BF₄] have been analyzed in a frequency range 4000-400 cm^-1. Substantial changes in a temperature evolution of internal modes of both morpholinium cations and [BF₄]^- anions are due to the freezing of these moieties motions below 117 K. A dynamic nonequivalence of protons involved in N-H?F and N-H?O hydrogen bonds has been detected. Heat capacity anomalies around structural phase transitions detected by AC calorimetry (117 and 153 K) have been evaluated and described. A molecular mechanism of the phase transitions is discussed on the basis of presented results. Copyright

Full text of DOI:10.1016/j.chemphys.2011.01.002

Chemical Physics 381 (2011) 11–20
Contents lists available at ScienceDirect
Chemical Physics
journal homepage: www.elsevier.com/locate/chemphys
Vibrational and thermodynamic properties and molecular motions in the
1
incommensurate crystal of morpholinium tetrafluoroborate studied by H NMR
a
b
c
d
e
M. Owczarek a, R. Jakubas a, , G. Bator , A. Pawlukojc , J. Baran , J. Przesławski , W. Medycki
⇑
´
a Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland
b Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland
c Institute of Low Temperature and Structural Research of the PAS, Okòlna 2, 50-422 Wrocław, Poland
d Institute of Experimental Physics, University of Wrocław, Max Born Sq. 9, 50-204 Wrocław, Poland
e
´
Institute of Molecular Physics, PAS, Smoluchowskiego 17, 60-179 Poznan, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Dynamics of molecules in morpholinium tetrafluoroborate, [NH2(C2H4)2O][BF4], has been studied in a
wide temperature range by means of AC calorimetry, 1H NMR and infrared spectroscopy. Simultaneously,
DFT (density functional theory) calculations of the molecular structure and normal vibration frequencies
have been performed by using various functionals for initial structural data of the low temperature phase.
Infrared spectra of polycrystalline [NH2(C2H4)2O][BF4] have been analyzed in a frequency range 4000–
400 cmꢀ1. Substantial changes in a temperature evolution of internal modes of both morpholinium
cations and [BF4]ꢀ anions are due to the freezing of these moieties motions below 117 K. A dynamic non-
equivalence of protons involved in N–Hꢁ ꢁ ꢁF and N–Hꢁ ꢁ ꢁO hydrogen bonds has been detected. Heat capac-
ity anomalies around structural phase transitions detected by AC calorimetry (117 and 153 K) have been
evaluated and described. A molecular mechanism of the phase transitions is discussed on the basis of
presented results.
Received 23 August 2010
In final form 5 January 2011
Available online 9 January 2011
Keywords:
AC calorimetry
1H NMR
Infrared
DFT calculation
Morpholinium tetrafluoroborate
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
MHSO4 [13], MClO4 [14] and MReO4 [15] were synthesized. They
are closely related to the compound under investigation,
Investigations of compounds built up of organic cations and
either inorganic or organic anions are of value as they provide inter-
esting supramolecular networks. The characteristic feature of these
networks is that the moieties can participate in hydrogen bond sys-
tems. Hydrogen bonds have been recognized as the most powerful
force to generate supramolecular assemblies of molecules. In such
systems we can combine desirable properties of inorganic materials
with high non-linear optical activity of organic substances [1–5].
The most interesting feature in these connections is the presence
of non-linear optical activity (NLO) and ferroelectric properties. In
the case of morpholinium connections with inorganic moieties
the NLO effect was found in e.g. morpholinium dihydrogenphos-
phate [6]. Numerous quite simple (1 : 1) ionic connections of mor-
pholinium cations with organic acid anions [7,8], mainly with
substituted benzoic acid (e.g. 4-hydroxybenzoic, 2-chloro-4-nitro-
benzoic and 2,3-dicyanohydroquinone [9–11]), are also character-
ized by NLO properties at room temperature. Recently, the first
series of thermotropic ionic liquid crystals based on the morphol-
inium core has been reported [12]. Several examples of simple ionic
morpholinium (M) complexes with tetrahedral AB4-type anions:
[NH2(C2H4)2O][BF4], taking into account their polymeric organic
network.
The
previous
calorimetric
(DSC)
measurements
of
[NH2(C2H4)2O][BF4] disclosed quite a complex sequence of phase
transitions (PTs) at low temperature [16]:
The phase transition (PT) I ? II appears continuous, whereas
the low temperature one, II ? III, exhibits discontinuous character.
The latter PT is accompanied by a significant transition entropy va-
lue, which is explained in terms of an order–disorder mechanism.
Single crystal X-ray diffraction studies showed that the disordered
high temperature orthorhombic phase I and the low temperature
ordered orthorhombic phase III are separated by a narrow inter-
mediate phase II, which appeared to be modulated incommensura-
bly [16,17]. The II ? III phase transition is accompanied by a
tripling of the unit cell volume (cIII = 3cI). This phase transition is
classified as a nonferroelectric one. Nevertheless, such a sequence
⇑
Corresponding author. Fax: +48 713282348.
E-mail address: rj@wchuwr.chem.uni.wroc.pl (R. Jakubas).
0301-0104/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemphys.2011.01.002

12
M. Owczarek et al. / Chemical Physics 381 (2011) 11–20
of phase transitions is typical of ferroelectric materials with a
‘‘lock-in’’ discontinuous transition [18,19]. The crystal structure
of [NH2(C2H4)2O][BF4] in the phase I consists of morpholinium cat-
ions, forming one-dimensional hydrogen-bond network, and
highly disordered tetrafluoroborate anions (see Fig. 1). The dynam-
ics of anionic moieties was suggested to contribute to the II ? III
PT mechanism. The role of morpholinium cations in the mecha-
nism of both transitions is unclear. Nevertheless, the isotopic effect
is visible as a shift of temperature of both phase transitions by
about 7 K. It means that N–Hꢁ ꢁ ꢁF and N–Hꢁ ꢁ ꢁO hydrogen bonds
may be involved in the dynamics of molecules in the crystal struc-
ture. In order to throw more light on the molecular mechanism
of the phase transitions we decided to carry out precise AC
calorimetric measurements, vibrational and proton magnetic reso-
nance (1H NMR) studies in a wide temperature range on the
[NH2(C2H4)2O][BF4] crystal.
spectrometer over the wavenumber range 3500–80 cmꢀ1 at room
temperature. Grams/368 Galactic Industries program was used
for numerical fitting of the experimental data. Gaussian functions
were used for fitting the infrared bands.
A computer controlled AC calorimeter (Sinku-Riko ACC-1 M/L)
has been used. Thermal response was detected by a chromel-
constantan thermocouple using a lock-in amplifier. The AC calori-
metric measurements were performed with a chopping frequency
of 0.8 Hz during heating and cooling at the rate of 0.05 K/min.
T1 (1tH NMR) relaxation time measurements were performed on
a ELLAB TEL-Atomic PS 15 spectrometer working at the frequency
of 25 MHz. The temperature of the sample was automatically sta-
bilized by a UNIPAN 650 temperature controller operating with a
Pt-100 sensor. The T1 relaxation times were determined by using
a saturation method. The powdered sample of the crystal was de-
gassed under pressure of 10ꢀ5 Torr (1.32 ꢂ 10ꢀ3 bar) and sealed
under vacuum in glass ampoules.
2. Experimental
3. Structural data
Colorless and transparent single crystals of [NH2(C2H4)2O][BF4]
were obtained by a stoichiometric reaction (1 : 1) of morpholine
and fluoroboric acid. The chemical formula of the compound
was confirmed by an elemental analysis (found: C, 26.56%; N,
7.69%; H, 5.47%; calculated: C, 26.67%; N, 7.78%; H, 5.56%). The
obtained crystals were twice recrystallized from a methanol
solution. Single crystals were grown by slow evaporation of the
solution at constant room temperature (RT). A deuterated com-
pound [ND2(C2H4)2O][BF4] was obtained by dissolution of [NH2-
(C2H4)2O][BF4] crystals in D2O and evaporation in vacuum condi-
tions. The material was twice recrystallized from D2O.
Infrared spectra of [NH2(C2H4)2O][BF4] were measured in a wide
temperature range (at cooling runs) for mulls in Nujol and Fluorol-
ube (only RT) with a Bruker IFS-88 spectrometer over the wave-
number range 4000–500 cmꢀ1 with a resolution of 1 cmꢀ1. APD
Cryogenics with closed cycle helium cryodyne system was used
for temperature-dependent studies. The temperature of the sample
was maintained at the accuracy of 0.1 K. Powder FT-Raman spectra
were measured with a FRA-106 attachment to the Bruker IFS-88
We have theoretically calculated molecular geometries and IR
frequencies for the isolated [BF4]ꢀ anion and the morpholinium
cation as well as for a solid state model of the [NH2(C2H4)2O][BF4]
salt. We have used DMol3 program [20,21] from Materials Studio
package [22] for the Becke–Lee–Yang–Parr correlation functional
(BLYP) [23,24] and dnp (double numerical and polarization on hea-
vy and hydrogen atoms) basis sets as implemented in DMol3. An
initial data set for the calculations was taken from crystallographic
data (80 K). For the system of two hundred and fifty-two (252)
atoms in the crystallographic unit cell seven hundred and fifty-
three (753) frequency modes have been obtained in which six hun-
dred and twelve (612) describe normal vibrations of twelve (12)
[BF4]ꢀ anions and of twelve (12) morpholinium cations, and one
hundred and forty-one (141) describe translational and rotational
modes of twelve anion-cation pairs of the [NH2(C2H4)2O][BF4] salt
in the phase III. The corresponding modes of morpholinium tetra-
fluoroborate were defined by means of internal coordinates
according to [25]. Theoretically predicted frequencies were not
scaled.
A comparison of experimental bond lengths and angles with the
calculated ones in the crystalline phase (80 K) as well as for the iso-
lated molecule is presented in Table 1. From these data, it clearly
follows that the tetrahedral geometry of anions is somewhat differ-
ent from that calculated for the crystal. B–F distances differ by ca.
0.03 Å. The geometry of [NH2(C2H4)2O]+ cations obtained from
X-ray measurements is slightly distorted. C–C and C–N bond
lengths are in good agreement with the calculated data for the
crystal. The most visible change is seen in C–O bond length which
is almost the same as for isolated molecules. The average differ-
ence in torsion angles is 1.5° (max. ꢃ5° for C–C–O–C) which can
be a consequence of a formation of N–Hꢁ ꢁ ꢁO and N–Hꢁ ꢁ ꢁF hydrogen
bonds.
4. Selection rules
In the high temperature phase the crystal is orthorhombic and
belongs to the Pnam space group. The unit cell contains four formal
molecules (Z = 4). Taking into account only these data, both cations
and anions should occupy Cs(xy) sites. The Cs site symmetry of the
morpholinium cations follows from the X-ray data. The following
atoms: N1, O1, H1C and H1D occupy the m symmetry plane being
parallel to the ab(xy) crystallographic plane. Internal modes of this
cation may be classified as A0(23) and A00(19). Due to the correlation
field (Davydov) splitting each internal mode splits into four
(A0 ? Ag+B1g+B2u+B3u; A00 ? B2g+B3g+Au+B1u) unit cell modes (see
Table 2).
Fig. 1. Hydrogen bonds network in [NH2(C2H4)2O][BF4] (phase I).

M. Owczarek et al. / Chemical Physics 381 (2011) 11–20
13
Table 1
Calculated structure (distances in Å and angles in °) of the isolated molecule, in the crystal and from X-ray single crystal diffractional studies of [NH2(C2H4)2O][BF4].
Coordinates
Calculated DMol3 (BLYP/dnp)
Isolated molecules
1.434
Observed
80 K
Crystal
B–F
1.428, 1.420, 1.428
1.431, 1.417, 1.429
1.417, 1.421, 1.416
1.423, 1.441, 1.423
109.81, 111.11, 109.84
109.49, 109.81, 109.78
110.40, 109.45, 110.29
109.43, 108.47, 108.09
1.452, 1.460
1.455, 1.453
1.459, 1.452
1.524, 1.519
1.519, 1.525
1.397, 1.393, 1.399
1.400, 1.392, 1.392
1.395, 1.394, 1.393
1.396, 1.410, 1.396
110.37, 111.11, 110.60
109.32, 109.46, 109.86
110.30, 109.69, 110.21
109.00, 108.14, 108.03
1.434, 1.444
1.435, 1.432
1.440, 1.435
1.516, 1.517
1.514, 1.516
F–B–F
109.47
O–C
C–C
N–C
1.435
1.530
1.540
1.519, 1.523
1.509, 1.513
1.515, 1.515
1.499, 1.496
1.509, 1.512
1.496, 1.501
1.512, 1.509
1.500, 1.495
O–C–C
C–C–N
110.81
110.78
108.61
108.7
112.14
111.63
ꢀ56.36
52.12
110.75, 109.74, 110.19
110.05, 110.48, 110.66
109.21, 109.31, 109.84
109.29, 109.69, 108.95
111.12, 111.27, 111.06
111.17, 110.82, 111.25
ꢀ56.90, ꢀ56.11, ꢀ57.48
55.17, 53.80, 56.15
ꢀ56.03, ꢀ55.27, ꢀ55.79
57.95, 58.86, 57.49
ꢀ61.08, ꢀ67.67, ꢀ60.75
60.65, 61.34, 60.60
111.08, 110.10, 110.34
110.22, 110.66, 110.59
108.65, 109.05, 109.12
109.29, 109.38, 108.82
110.43, 110.45, 110.09
110.75, 110.50, 110.90
ꢀ58.22, ꢀ57.26, ꢀ58.07
56.30, 54.94, 57.02
ꢀ56.74, ꢀ55.96, ꢀ57.07
58.12, 59.27, 58.66
ꢀ60.55, ꢀ62.45, ꢀ60.89
61.00, 61.49, 60.49
C–N–C
C–O–C
O–C–C–N
C–C–N–C
C–N–C–C
N–C–C–O
C–C–O–C
C–O–C–C
ꢀ52.17
56.51
ꢀ63.51
63.4
Assuming that the tetrafluoroborate anions occupy the Cs
sites one expects the following correlation for its internal
modes:
however, of three (Z0 = 3) formal molecules; i.e. three [BF4]ꢀ tetra-
hedral anions and three morpholinium cations. Each of them occu-
pies the general C1(4) sites. For each formal molecule (i.e. for each
anion and each cation) its normal modes split into four unit cell
modes (A + B1 + B2 + B3). In this approach, the dynamical coupling
between the normal modes of various components of the asym-
metric unit are negligible. The results of the formal fundamental
modes (k = 0) are given in Table 3.
m m m m4(F2) ?
1(A1) ? A0; 2(E) ? A0+A00; 3(F2) ? 2A0+A00;
2A0+A00. Additionally, each component of the internal modes splits
into four unit cell modes (see above). The expected splitting are gi-
ven in Table 2. However, the X-ray data suggest a disorder for these
anions. It is suggested that only F1 atoms occupy the Cs site. In such
a case, the real local symmetry for tetrafluoroborate anions will be
C1 and the correlation (unit cell or Davydov) type splitting cannot
occur. In this case, it is impossible to perform the formal classifica-
tion of the unit cell modes either.
5. IR and Raman spectra
In the low temperature phase III the title crystal belongs to the
Infrared spectra of polycrystalline [NH2(C2H4)2O][BF4] in a
wavenumber range between 4000 and 500 cmꢀ1 at 10, 80 (phase
III) and 300 K (phase I) and Raman spectra (300 K) are presented
P212121ðD4Þ space group of the orthorhombic system. The unit
2
cell contains 12 formal molecules, the asymmetric unit consists,
Table 2
The formal classification of the fundamental modes (k = 0) for the [NH2(C2H4)2O][BF4] crystal in the phase I (space group Pnam Z = 4). It is assumed that both ions occupy the
Cs(xy) sites. According to the X-ray data, [BF4]ꢀ anions occupy the C1 sites. In such a case, the classification of [BF4]ꢀ modes is not possible.
Sitea
UCG
D2h
Lattice modes
Internal modes
[NH2(C2H4)2O]+
Selection rulesb
[BF4]ꢀ
Cs(xy)
Ac
Lib
Tran
m
1(A1)
m
353
2(E)
m
3(F2)
m
524
4(F2)
IR
Raman
769c
1075
Ag
B1g
0
0
2
2
4
4
23
23
1
1
1
1
2
2
2
2
i
i
xx, yy, zz
xy
A0?
B2u
B3u
B2g
B3g
1
1
0
0
2
2
4
4
3
3
2
2
23
23
19
19
1
1
0
0
1
1
1
1
2
2
1
1
2
2
1
1
y
z
i
i
i
zx
yz
i
A00
?
Au
B1u
0
1
4
4
2
1
19
19
0
0
1
1
1
1
1
1
i
x
i
i
a
Abbreviations: Site – group of site symmetry; UCG – unit cell group (equivalent to the factor group); Ac – acoustic modes; Lib. – librational lattice modes; Tran – translational
lattice modes.
b
i – inactive modes.
c
The wavenumbers of the internal modes for the isolated [BF4]ꢀ anion of the Td symmetry (according to [26]).

14
M. Owczarek et al. / Chemical Physics 381 (2011) 11–20
Table 3
The formal classification of the fundamental modes (k = 0) for [NH2(C2H4)2O][BF4] for the low temperature phase (orthorhombic system, space group P212121ðD42Þ; Z = 12; Z0 = 3).
Lattice modesa
Internal modesb
Selection rules
[NH2(C2H4)2O]+
[BF4]ꢀ
D2
Ac
Lib
Tran
m
1(A1)
m
353
2(E)
m
3(F2)
m
524
4(F2)
IR
Raman
769c
1075
A
0
1
1
1
18
18
18
18
18
17
17
17
42 (126)
42 (126)
42 (126)
42 (126)
1 (3)
1 (3)
1 (3)
1 (3)
2 (6)
2 (6)
2 (6)
2 (6)
3 (9)
3 (9)
3 (9)
3 (9)
3 (9)
3 (9)
3 (9)
3 (9)
i
xx, yy, zz
B1
B2
B3
z
y
x
xy
zx
yz
a
Abbreviations: Ac – acoustic modes; Lib – librational modes; Tran – translational modes.
Outside of brackets – the number of modes for one molecule in the asymmetric unit; in brackets – the number of modes for asymmetric unit (i.e. for three molecules
b
(ions)).
c
The wavenumbers of the internal modes for the isolated [BF4]ꢀ anion of the Td symmetry (according to [26]).
Fig. 2. The infrared spectra of the powdered [NH2(C2H4)2O][BF4] sample in Nujol (10 and 300 K) and in Fluorolube (300 K) and the Raman spectrum at 300 K.
Fig. 3. The infrared spectra of the powdered deuterated [ND2(C2H4)2O][BF4] sample in Nujol (10 and 300 K) and in Fluorolube (300 K) and the Raman spectrum at 300 K.

M. Owczarek et al. / Chemical Physics 381 (2011) 11–20
15
Table 4
Comparison of the selected experimental and calculated IR and Raman spectral data for [NH2(C2H4)2O][BF4] and for the deuterated homologue.
Calculated DMol3 (blyp/dnp)
Observed
Tentative assignments
Isolated ions
Crystal
Non-deuterated
Deuterated
IR
IR
Raman
300 K
Raman
300 K
80 K
80 K
10 K
80 K
300 K
300 K
3247(s)
3208(w)
3170(m)
3156(vw)
3084(sh)
3074(s)
3247(m)
3211(w)
3169(w)
3156(sh)
3081(m)
3071(sh)
3040(w)
3237(m)
3237(vw)
3233(vw)
(NHþ2
or
)
3380
3319
3306
3301
3251
3132
3129
3128
m
3162(w)
3082(w)
3083(w)⁄
N–Hꢁ ꢁ ꢁY
3071(vw)
3041(vw)
(Y = O, F)
3043(s)
2422(w)
2404(w)
2313(vw)
2280(vw)
2255(vw)
2229(sh)
2417(vw)
2403(vw)
2310(vw)
2280(vw)
2256(vw)
2227(vw)
m
(NDþ2
or
)
N–Dꢁ ꢁ ꢁY
(Y = O, F)
1622
1373
1280
968
1624
1601
1595
1408
1407
1399
1246
1242
1233
988
1596(vw)
1580(w)
1594(vw)
1580(m)
1587(vw)
1581(sh)
d(NHþ2
)
1581(w)
1583(vw)⁄
1432(w)⁄
1359(vw)⁄
1444⁄(m)
1436⁄(sh)
1444(sh)
1433(vw)
1442(vw)
1433(vw)
x
(NHþ2
)
s
(NHþ2
)
1357⁄(m)
1355(vw)
1216(vw)
1357(vw)
1222(vw)
1217(vw)
981
1221(vw)
1221(w)
1221(vw)
m(C–N)
966
1167(vw)
1158(w)
1153(w)
1123(sh)
d(NDþ2
)
1156(vw)
1148(vw)
1127(vw)
1121(sh)
1111(vw)
1096(vw)
847
900
885
882
1126(s)
1118(sh)
1126(s)
1118(sh)
1120(vw)
1117(vw)
1112(vw)
(NHþ2
)
1109(sh)
1097(vs)
1084(vs)
q
1097(vs)
1097(vs)
1084(vs)
1077(vs)
1071(vs)
1012
1012
1012
742
1026
1021
1018
1014
1010
1007
1003
999
994
795
792
790
1085(vw)
1075(vw)
1071(vw)
1078(sh)
1070(vs)
1080(vs)
1067(vs)
1080(vw)
1072(vw)
1065(vw)
1060(vw)
1069(sh)
1053(vs)
1061(vs )
1052(vs)
1059(vs)
1054(vs)
1050(sh)
m3(BF4)
1045(sh)
1042(vs)
1045(vw)
1043(vs)
900(m)
1042(vw)
m
s
(C–N)
(NDþ2
)
986(vw)
987(vw)
980(vw)
1154
1021
1013
963
1071
1063
1057
1039
1036
1034
1033
1031
1027
867
900(sh)
898(m)
894(sh)
893(s)
895(s)
894(sh)
893(sh)
888(sh)
885(m)
882(w)
877(m)
887(s)
887(s)
q(CH2)
884(vw)
881(w)
877(s)
883(sh)
880(sh)
876(m)
884(vw)
873(vw)
883(vw)
865(vw)
868(s)
867(s)
866(m)
869(w)
859(w)
863
859
840
857(vw)
840(vw)
837
844(vw)
844(vw)
840(vw)
m(C–C)
837
834
(NDþ2
)
817(vw)
806(vw)
772(vw)
768(vw)
815(vs)
q
707
479
732
730
729
508
506
769(vw)
769(vw)
769(vw)
527(vw)
767(w)
770(w)
m1(BF4)
532(vw)
528(vw)
532(vw)
528(vw)
533(vw)
527(vw)
528(vw)
(continued on next page)

16
M. Owczarek et al. / Chemical Physics 381 (2011) 11–20
Table 4 (continued)
Calculated DMol3 (blyp/dnp)
Observed
Tentative assignments
Isolated ions
Crystal
Non-deuterated
Deuterated
IR
Raman
300 K
IR
Raman
300 K
80 K
479
80 K
10 K
80 K
300 K
300 K
502
498
494
491
489
487
486
524(vw)
520(w)
518(sh)
513(vw)
520(w)
518(sh)
512(vw)
521(vw)
517(sh)
513(vw)
520(vw)
520(vw)
513(vw)
519(vw)
m4(BF4)
479
vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder; ⁄ in Fluorolube
m
– stretching; d – scissoring;
q
– rocking; s – twisting; x – wagging.
a
b
3260
3240
3220
3200
3180
3160
3140
3120
3100
3080
3060
3040
3020
3000
290K
250K
230K
180K
170K
160K
150K
140K
130K
120K
115K
110K
105K
100K
III
II
I
90K
80K
50K
10K
0
50 100 150 200 250 300
3300 3250 3200 3150 3100 3050 3000
wavenumber [cm-1 ]
Temperature [K]
Fig. 4. (a) Temperature evolution of the IR spectra in the
m
(NHþ2 ) vibration region (3300–3000 cmꢀ1) and (b) the wavenumber positions of these modes between 10 and 300 K.
905
a
b
290K
250K
900
230K
180K
170K
895
890
885
880
875
870
865
160K
150K
140K
130K
120K
115K
110K
105K
100K
90K
80K
50K
10K
III
II
I
910 900 890 880 870 860
0
50 100 150 200 250 300
wavenumber [cm-1]
Temperature [K]
Fig. 5. (a) Temperature evolution of the IR spectra in the m m(CH2)
(CH2) vibration region (910–860 cmꢀ1) and (b) temperature dependence of the wavenumber positions of the
modes.

M. Owczarek et al. / Chemical Physics 381 (2011) 11–20
17
a
b
534
290K
250K
230K
180K
170K
160K
150K
140K
130K
532
530
528
526
120K
110K
III
II
I
115K
105K
520
518
516
514
512
510
100K
90K
80K
50K
10K
540 535 530 525 520 515 510 505
0
50 100 150 200 250 300
wavenumber [cm-1]
Temperature [K]
Fig. 6. (a) Temperature evolution of the IR spectra in the m
4(BF4) vibration region (540–505 cmꢀ1) and (b) temperature dependence of the wavenumber positions of these
modes.
in Fig. 2. Fig. 3 shows infrared and Raman spectra of deuterated
[ND2(C2H4)2O][BF4] at room temperature. The IR spectra in a
frequency range 3000–2800, 1500–1350, 750–700 cmꢀ1 are shown
as a dotted line as Nujol bands appear in these regions. To cover
these regions we measured infrared spectra of [NH2(C2H4)2O]-
[BF4] (normal and deuterated) in Fluorolube mulls, however, only
at 300 K. The experimental and calculated frequencies of
[NH2(C2H4)2O][BF4] bands and experimental frequencies of the
deuterated homologue are collected in Supplementary Table S1.
The selected data related to wavenumber regions analyzed below
are shown in Table 4.
ing and scissoring vibration bands of the (NHþ2 ) group occur in re-
gions characteristic of other protonated secondary amines. From
Fig. 3 one can see that for deuterated [ND2(C2H4)2O][BF4] in a
wavenumber region 3300–3000 cmꢀ1 almost all bands of (NH2þ)
modes disappear and the other ones emerge in 2500–2100 cmꢀ1
region. The deuterium substitution causes frequency shift by a fac-
pffiffiffi
tor of 1.37, which is almost equal to 2 of the isotopic shift. The
same situation occur in a case of other (NDþ2 ) modes.
An analysis of the evolution of IR spectra performed in a wave-
number region between 3300–3000 cmꢀ1 assigned mainly to the
stretching vibrations N–Hꢁ ꢁ ꢁO and N–Hꢁ ꢁ ꢁF (Fig. 4) shows that
some components change their positions or split into several com-
ponents at temperature close to Tc = 117 K. It should be noted that
mentioned above changes are more significant at the low temper-
ature phase transition than at the other one. The modes at 3028
and 3082 cmꢀ1 are bands the most sensitive to the II ? III phase
transition. On cooling these bands split into some components just
below 117 K. In a case of the band at 3237 cmꢀ1 the splitting has
occurred below I ? II phase transition.
An analysis of the isolated bands arising from some internal
vibrations (for instance the band at 769 cmꢀ1 in IR and Raman
spectra arising from m1(BF4)) clearly shows lack of the correlation
field (or Davydov) splitting in powder spectra of the title crystal.
Therefore, in the discussion of the internal vibrations it is enough
to take into account only the site approach for each ion. In the case
of the tetrafluoroborate ions it is clearly seen that the anion sym-
metry is not tetrahedral (Td; as it was suggested by Szklarz et al.
[16]) but either Cs or C1. One should mention that on the basis of
vibrational spectra measured for the powder sample these two
cases are impossible to distinguish.
Fig. 5 presents an evolution of IR spectra and temperature
dependence of the positions of the bands in a region assigned to
m
(CH2) vibrations (910–860 cmꢀ1). At low temperature eight bands
Tentative assignments of the observed bands arising from cat-
ionic vibrations are based on a comparison with a morpholine IR
spectrum [27] and similar heterocyclic compounds [28–31]. The
work of Bates et al. [32] and [26] were used to assign the [BF4]ꢀ an-
ion vibrations. The assignments of the (NHþ2 ) group were made tak-
ing into account results reported for protonated secondary amines
[33].
can be assigned to m(CH2) vibrations and one band to C–C (at
868 cmꢀ1) vibration. Again, the remarkable changes in positions
of the bands are observed at ca. 117 K. Some bands appear just be-
low Tc (bands at 895, 893 cmꢀ1), whereas the band at 880 cmꢀ1
splits into two components. The characteristic feature of the tem-
perature behavior of some bands (e.g. 888 and 895 cmꢀ1 at 300 K)
is a relatively large temperature coefficient of the change in their
positions. However, no discontinuity in the temperature depen-
dence of their positions is observed at 153 K (I ? II PT).
From data in Table 4 it clearly follows that the assignments of
[BF4]ꢀ vibration bands and most of the cationic modes are well
consistent with the calculated frequency values. The most signifi-
Fig. 6 shows an evolution of IR spectra vs temperature in a
wavenumber region between 540–505 cmꢀ1. The bands in this
wavenumber range arise from m4 vibrations of the [BF4]ꢀ anions
(see Table 4). At the phase transition temperature (transition to
the phase III) the band at 527 cmꢀ1 shifts towards high wavenum-
bers (528 cmꢀ1), whereas the band at 519 cmꢀ1 splits into two
components (517 and 520 cmꢀ1). Moreover, two new bands appear
cant divergences are seen in a case of (NHþ2 ) and
m
(C–N) modes.
m
(C–N) stretching vibrations, in comparison with the calculated
values, are shifted towards high frequencies. To assign correctly
modes corresponding to (NHþ2 ) vibrations, IR and Raman spectra
of deuterated [ND2(C2H4)2O][BF4] were used. The protonated
amine group is involved in one N–H(1D)ꢁ ꢁ ꢁO(1) and at least two
various N–Hꢁ ꢁ ꢁF hydrogen bonds: (N1–H(1C)ꢁ ꢁ ꢁF(6) and N(1)–
H(1C)ꢁ ꢁ ꢁF(3) or N1–H(1C)ꢁ ꢁ ꢁF(6)a and N(1)–H(1C)ꢁ ꢁ ꢁF(3)a, respec-
tively (Table 4 in [16]), which causes a shifting of the stretching
vibration bands towards low frequencies. Rocking, wagging, twist-
over the phase III at 532 and 512 cmꢀ1
.
The appearance of the significant changes in IR spectra on
changing temperature in wavenumber ranges corresponding to
the vibrations of the (NH2þ) groups as well as of the [BF4]ꢀ anions

18
M. Owczarek et al. / Chemical Physics 381 (2011) 11–20
a
b
600
500
400
300
200
100
cp
[J/mol K]
cp
400
[J/mol K]
300
III
II
I
III
II
I
200
100
0
100
120
140
160
180
200
100
120
140
160
180
200
T[K]
T[K]
Fig. 7. Temperature dependence of the specific heat of [NH2(C2H4)2O][BF4] compound (a) and excess specific heat (b). The red line (in part (b)) gives a provisional separation
of two transitions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
confirms the supposition that the anions and N–Hꢁ ꢁ ꢁY hydrogen
bonds formed by them play a crucial role in the mechanism of
the phase transitions in the [NH2(C2H4)2O][BF4] crystal.
Thermal parameters for the second-order transition (153 K) to
the IC phase were estimated: an excess entropy
S ꢄ 7.13 J/
mol ꢁ K, and an excess enthalpy H = 983.4 J/mol. The values ob-
tained are quite high as for the N-IC transition (e.g. the S value
D
D
D
is comparable with the value reported for crystals of the A2BX4-
type with tetramethylammonium cations [34]). The critical behav-
ior of the cp-anomaly at this transition has been analyzed with the
preasymptotic 3D XY model renormalization group expression gi-
ven by Bagnuls and Bervillier, [35]:
6. Thermodynamic properties
Fig. 7a shows temperature dependence of the specific heat dur-
ing cooling in a limited temperature range. Two anomalies con-
nected with the previously mentioned phase transitions were
detected at temperatures 117 K and 153 K. The AC calorimeter
can be used only for qualitative studies of first-order transitions.
Nevertheless, we were able to estimate a very small temperature
hysteresis, of the order of 0.322 K, for the ‘‘lock-in’’ first-order tran-
sition at 117 K. It should be taken into account that absolute values
of the specific heat could be determined in relation to a reference,
which in our case was a sapphire sample. The anomalous specific
a
D1
D
cp ¼ Aꢅjtjꢀ ð1 þ D1ꢅjtj þ DꢅjtjÞ þ Bcr
ð1Þ
2
where superscripts denote parameter values for temperature
above and below the transition point, Ti; t = (T ꢀ Ti)/Ti is a reduced
temperature; A is a critical amplitude;
a
-a critical exponent; D1ꢅ
and Dꢅ2 are the first and second-order correction terms; Bcr-a critical
constant; and D1 is a subcritical exponent.
In Fig. 8a the anomaly of the heat capacity
Dcp/R at the normal-
heat excess
ground approximated by a polynomial. It is rather difficult to split
the cp(T) run into two parts corresponding to two phase transi-
tions. In order to get more reliable heat flow values, the data re-
lated to the incommensurate phase (IC) and to the I ? II
transition were fitted using an exponential function extrapolated
to the background line (see Fig. 7b).
D
cp was determined by a subtraction of a lattice back-
incommensurate transition (circles) and the 3D XY fit (solid line)
are shown. The procedure of fitting, similar to that used by Haga
D
et al. [36], was applied. The exponent
a
=ꢀ 0.007, the critical
amplitude ratio Aꢀ=Aþ ¼ 0:869; D1þ ¼ ꢀ0:045; D1ꢀ ¼ ꢀ0:041; D1
¼
a
þ
0:524; Dꢅ2 ¼ ꢀ0:002 and a dimensionless factor: RBþ ¼ A jD1 j
þ
D
1
ꢀ1
B
ꢄ ꢀ0:944 are quite close to the theoretically expected for 3D
cr
XY model. For points above Ti a slight discrepancy between fitting
a
b
-2
15
cp/R
( cp)
2x10-3
12
9
II
III
1x10-3
6
3
0
-0.15
-0.10
-0.05
0.00
0.05
0.10
144
147
150
T [K]
153
156
t = (T-T)/T
i
i
Fig. 8. (a) Excess specific heat
The metastable limit is also shown.
Dcp/R near the normal-incommensurate transition (filled circles) and fit with the Eq. (1) (solid line) and (b) temperature dependence of (D .
cp)ꢀ2

M. Owczarek et al. / Chemical Physics 381 (2011) 11–20
19
In this equation
x
represents a resonance angular frequency, C-
and experimental points can be noted. It should be also pointed
relaxation constant and sc is a correlation time. The temperature
dependence of the correlation time is described by the Arrhenius
that the transition is not far (
(Fig. 8b).
DT = 0.65 K) from the tricritical point
law: sc = s0exp(Ea/RT), where s0 is a correlation time at the limit infi-
7. Proton magnetic resonance (1H NMR) studies
nite temperature, Ea-the height of the barrier, R-gas constant. This
gives three sets of parameters for the three temperature regions
of T1 analyzed. The values of Ea and s0 estimated using Eqs. (2)
and (3) are shown in Table 5. The data relate to two assumed relax-
ation processes of the (NHþ2 ) protons in the morpholinium cation.
The activation energy values, evaluated for the high temperature re-
gions of T1(1000/T), are collected in Table 5 as well.
It should be noted that the estimated activation energy values
at high temperature (above 153 K, 9.62 kJ/mol) are very close to
that found at low temperature (below 117 K, 10.04 and 9.46 kJ/
mol). It may be explained by the fact that mechanism of both phase
transitions, at 117 and at 153 K, is assigned to the changes in
molecular dynamics of anions, which seems to affect insignifi-
cantly the cationic internal motions.
The temperature dependence of the spin–lattice relaxation time
may be divided into three parts separated by two phase transition
temperatures, 117 K and 153 K (Fig. 9). Below 117 K two well sep-
arated minima of T1 relaxation time are observed with comparable
depths: 0.24 and 0.27 s at 94 K and 111 K, respectively. In the tem-
perature range 117–153 K (IC phase) a slope of the curve found
seems to be unexpectedly lower than that detected at low temper-
ature. Above 153 K (1000/T < 6.5 Kꢀ1) only the left side of the min-
imum of T1 relaxation time is reached with nearly the same slope
as for the other minimum found at low temperature. It should be
emphasized that both phase transitions are not accompanied by
any sudden discontinuities/jumps in the T1 value-its temperature
dependence reveals only slightly different slopes in particular tem-
perature ranges.
8. Discussion
It is obvious that below 117 K (1000/T > 8.5 Kꢀ1) two different
nonequivalent relaxation processes exist: one proton of the
(NHþ2 ) group is moving along the hydrogen bond of N–Hꢁ ꢁ ꢁO type,
whereas the other one along N–Hꢁ ꢁ ꢁF bond. It was assumed that
dynamics of both corresponding protons in three nonequivalent
cations and protons involved in the two types of hydrogen bonds
is comparable. Contributions of the two protons to the relaxation
phenomena are equal, which may be deduced from almost the
same depths of both minima on the dependence of T1 versus reci-
procal temperature. It means that the general formula for the reci-
procal value of T1 should be written as:
Our previous X-ray investigations on
a single crystal of
[NH2(C2H4)2O][BF4], carried out at several temperatures, suggested
that thermodynamical properties and the phase situation was re-
lated to a disorder of the [BF4]ꢀ groups [16]. Scanning calorimetry
(DSC) measurements allowed us, besides an establishment of a se-
quence of phase transitions, to roughly determine energetic effects
(entropic) for the I-st order phase transition, II ? III, at about
117 K. The dielectric studies indicated, moreover, a step-wise
freezing of some dipolar group motions at this temperature. Except
these aspects the molecular mechanism of both phase transitions
was unclear. It concerns mainly the I ? II transition of continuous
nature at 153 K. By definition this transition could not be analyzed
in details only on the basis of DSC results. Thus, the AC calorimetric
results, presented in this work, allow us to analyze more deeply
this mechanism as well as to give a thermodynamical characteris-
tics of this continuous transition. The estimated excess entropy for
1
T1
NO
1
NF 1
¼
þ
N T1O N T1F
ð2Þ
where T1O and T1F denote the relaxation times of the protons in N–
Hꢁ ꢁ ꢁO and N–Hꢁ ꢁ ꢁF hydrogen bridges, respectively; NO and NF de-
note the numbers of the relaxing protons of morpholinium cations
participating in hydrogen bonds, and N denotes the total number of
protons in morpholinium cations in the unit cell (Z = 12). In our case
Ni/N = 12/120. The temperature dependence of the spin–lattice
relaxation time T1i was analyzed by a BPP theory, using an equation:
this transition amounts to about
cates a clear ‘‘order–disorder’’ mechanism of the I ? II PT. The
magnitudes of Str for the low temperature transition obtained
D
S ꢄ 7.13 J/mol ꢁ K, which indi-
D
from either DSC or AC calorimetric measurements were compara-
ble, being of about 3.5–4.5 J/mol ꢁ K. Taking into account the four-
site model of the [BF4]ꢀ group disorder in the phase I, proposed on
ꢀ
ꢁ
1
T1i
sc
4sc
¼ C
þ
ð3Þ
1 þ x2s2c 1 þ 4x2s2c
the basis of the X-ray data, the total value of
DStr should equal to
D
Str(I ? II) +
D
Str(II ? III), which gives approx. ꢄ10.6–11.6 J/
mol ꢁ K. This value agrees well with the theoretical one for the
four-site model (Rln4 ꢄ 11.52 J/mol ꢁ K). This result indicates univ-
ocally that the dynamics of the [BF4]ꢀ groups is mainly responsible
for the phase transition mechanism at 153 K. This PT is probably
accompanied by a significant retardation of the [BF4]ꢀ anion reori-
entational motion, whereas at 117 K this motion is completely
locked.
Proton magnetic resonance studies revealed an unexpected pic-
ture of the possible molecular dynamics, particularly in the low
temperature phase (T < 117 K). If we took into account only
1000
T1 [s]
100
10
Ea =10.04kJ/mol
Ea =9.46kJ/mol
Ea=9.62kJ/mol
1
Table 5
0.1
0.01
Parameters characterizing detected proton relaxation calculated from the fit of Eq. (3)
to T1(T) experimental points below 117 K and the activation energies.
Ea=5.85kJ/mol
<117 K
117–153 K
5.85
>153 K
9.62
0
2
4
6
8
10
12
14
Ea [kJ/mol]
s0 [s]
10.04
9.46
1000/T[1/K]
0.8 ꢁ 10ꢀ13
1.5 ꢁ 10ꢀ14
3.16 ꢁ 109
–
–
–
–
Fig. 9. The temperature dependence of the spin–lattice relaxation time (1H NMR) of
[NH2(C2H4)2O][BF4]. Solid lines are results of theoretical fitting and dashed lines
denote the both components according to Eq. (2).
C [sꢀ2
]

20
M. Owczarek et al. / Chemical Physics 381 (2011) 11–20
crystallographic results this picture should be different. The X-ray
results indicated that the morpholinium cations were rather rigid
in the crystal structure, being involved in the one-dimensional
chain formed via N–Hꢁ ꢁ ꢁO hydrogen bonds. These chains were
additionally stabilized by N–Hꢁ ꢁ ꢁF hydrogen bonds to the [BF4]ꢀ
anions. Thus, we could expect rather weak temperature depen-
dence of T1 relaxation time. An appearance of two minima on T1
vs. temperature curve at low temperature proved that we dealt
with some proton dynamics in the phase III. There is no doubt that
merely the proton dynamics of the (NHþ2 ) groups, which are en-
gaged in the medium strong hydrogen bonds, could contribute to
the relaxation processes as the cation and anion motions are al-
ready frozen. These relaxation processes are indicated by the pres-
ence of two low temperature minima in the phase III. The lack of
temperature anomalies of T1 value around the phase transitions
at 153 and 117 K additionally confirms that the morpholinium cat-
ions are inactive in the mechanism of the structural phase transi-
tions. In this context the character of the dielectric response is
not fully understandable (see Fig. 7 in [16]). The dielectric response
reveals a relatively high change of the dielectric increment:
the pure ‘‘order–disorder’’ type (
D
Str ꢄ 7.1 J/mol K) and is gov-
erned by the reorientational motion of the [BF4]ꢀ groups. The
transition is close to the tricritical point.
3. The relaxation processes, observed in 1H NMR studies, may be
explained in terms of the proton dynamics in N–Hꢁ ꢁ ꢁO and
N–Hꢁ ꢁ ꢁF hydrogen bonds. They are believed to play a crucial
role in the phase transition mechanism at 117 and 153 K.
Acknowledgments
The calculations were performed on computers of Wroclaw
Center for Networking and Supercomputing, calculating Grant
No. 2006/5, and in Interdisciplinary Center for Mathematical and
Computational Modeling, Warsaw University, calculating Grant
No. G30-15. MATERIALS STUDIO package was used under POLAND
COUNTRY–WIDE LICENSE. This work was partially supported by
ZIBJ Dubna contract No. 04-4-1069-2009/2010.04.29.
Appendix A. Supplementary data
D
e0 ꢄ 4.5 at 117 K. If we do not relate it to the dynamics of the po-
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.chemphys.2011.01.002.
lar morpholinium cations, a change of the proton dynamics in
hydrogen bonds, which is correlated with the [BF4]ꢀ motions,
seems to be responsible for a change in a dipole moment of the
unit cell. As a consequence we should observe a jump in the dielec-
tric increment when the [BF4]ꢀ motion is frozen. It should be
emphasized that no dielectric relaxation process has been found
over the phase II up to 2 MHz. It means that the motion of the
[BF4]ꢀ anions is expected to be rather fast, thus dielectric relaxa-
tion has to be shifted up to the microwave or submillimeter fre-
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9. Conclusions
1. Significant changes with temperature in the IR spectrum, close
to 117 and 153 K, indicate that (NHþ2 ) and [BF4]ꢀ groups con-
tribute simultaneously to the phase transitions mechanism.
(NHþ2 ) and C–C stretching as well as the
m4(BF4) modes appear
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2. The AC calorimetric studies allow us to evaluate precisely ther-
modynamic parameters of the normal-incommensurate type
phase transition at 153 K. This transition may be classified as
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