On the polymorphism of solid cyclohexanol by adiabatic calorimetry and far infrared methods

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Submitted on 1 Jan 1990

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On the polymorphism of solid cyclohexanol by adiabatic

calorimetry and far infrared methods

J. Mayer, M. Rachwalska, E. Sciesinska, J. Sciesinski

To cite this version:

On

the

_polymorphism

of solid

_{cyclohexanol by}

adiabatic

calorimetry

and far infrared methods

J.

_Mayer

_(1),

M. Rachwalska

_(2),

E.

015Aciesi0144ska

₍₃₎

and J.

Sciesinski

₍₁₎

(1)

Henryk

Niewodnicza0144ski Institute of Nuclear

_{Physics, Radzikowskiego}

152, 31-342 Kraków,

Poland

(2)

Faculty

of

_{Chemistry, Jagellonian University,}

Karasia 3, 30-060 _Kraków, Poland

(3)

Institute of

_{Physics, Jagellonian University, Reymonta}

4, 30-059 Kraków, Poland

(Reçu

le 7

septembre

1989, révisé le 3

janvier

1990,

accepté

le 15

_janvier

₁₉₉₀₎

Résumé. 2014

Nous avons utilisé la calorimétrie

_adiabatique

et la

_{spectroscopie}

dans

_{l’infrarouge}

lointain afin de mieux

comprendre

le

polymorphisme

du

cyclohexanol

solide. Nous avons _pu

confirmer le

système

monotropique

des

_phases

II et III en nous basant sur les mesures

calorimétriques

effectuées entre 170 et 320 K et le bilan de

_{l’entropie.}

Nous avons vérifié les conditions de croissance de chacune des

phases

et les réversibilités des transitions de

_phases.

Nous

avons trouvé deux

façons

d’atteindre la

phase

III dont une _{nécessite le passage par} la

_phase

intermédiaire métastable MS. A côté de l’observation de la transition entre

_phases

III et _{II par}

méthodes

infrarouges,

la

_{technique calorimétrique}

nous a aussi

permis

d’observer la même transition

_{monotropique.}

Abstract. 2014

Adiabatic

_calorimetry

and FIR _spectroscopy were used to _get a better

_{understanding}

of the

_polymorphism

of solid

_{cyclohexanol.}

On the basis of calorimetric measurements

_performed

in the temperature range from 170 K to 320 K and the _entropy balance calculation, the

monotropic

set of

_phases

II and III was confirmed. The conditions in which different

phases

can

be grown and the

reversibility

of the

_{particular phase}

transitions were checked. Two different

ways for

obtaining phase

III, one

_{passing through}

an intermediate metastable

_phase

MS, were

demonstrated

_by

both methods.

_The

influence of the thermal treatment on the _apparent

stability

of

_phase

III was discussed.

In addition

to the

_previously

_reported

IR observation of the transformation from

_phase

III to _II, the same

_monotropic

transition was visualized in the

calorimetric measurement. Classification

Physics

Abstracts 65.40 - 78.30 1. Introduction.

Studies of the

_polymorphism

of

_cyclohexanol

were initiated in

_1929,

when

_Kelley

_[1]

found

two

_{crystal phases}

in this substance

_using

the calorimetric method. The most

_complete

information on the

_polymorphism

of solid

_cyclohexanol

was

_{given by}

Adachi et al.

[2] ;

it was

based on

_specific

heat vs.

_temperature

measurements in the

_temperature

_{range of} 14-320 K.

They

also gave a

_complete

review of earlier _papers on this

_subject.

_According

to

Adachi,

cyclohexanol,

below the

_{melting point}

of

_Tm

= 299.05

K,

has two stable

crystalline phases.

On

_heating,

the

_{low-temperature}

_phase

II transforms into

_phase

1 at a

_temperature

TII> _

1 = 265.5 K. Phase 1 is stable

(i.e.

has the lowest value of free

energy)

in the

_temperature

range between

TII, _> I and T.,

but on

_cooling

it can be

_{easily supercooled}

and below the

_glassy

type

transition at

_Tg

==150 K it forms a

_{glassy crystal}

with residual

_entropy

of 4.72 J/mol K. In the low

_temperatures

it is also

_possible

to _grow a

_{crystalline phase}

III,

metastable relative to

phase

II. At

_{TIII ->}

_{1 =} 244.8 K

_phase

III transforms into

_phase

I.

_Although

_phase

III is

metastable in the whole

_temperature

_{range, the authors did} not observe a transformation of

phase

III into

_phase

II.

Some time later Green and Griffith

_[3]

on the basis of dielectric and visual observations

reported

three solid

_phases

of

_{cyclohexanol :}

a

_{high-temperature}

rotational

_phase

_I,

which is

stable in the

_temperature

_{range from}

_{TII -+}

₌ 264.9 K to the

_{melting point T fi}

= 298.2

K,

_a

crystalline phase

II

_(1),

stable in the

_temperature

_{range from}

_{TIII -> II}

estimated as

_being

ca

210 K to

_{TII > I,}

and a

_{low-temperature}

_phase

_III,

stable below

_{TIII _>}

II

Apart

from

that,

they

found the transition from

_phase

III into

_phase

1

_(i.e.

the transition from overheated

_phase

III

to

_phase

_I)

at

_{TIll -+}

_{I =} 244 K.

_They

also observed the existence of an

_additional,

low-temperature

phase

which

_they

called «

pre-II

_», metastable with

regard

to both

_phase

II and

phase

III but more stable below ca 230 K than the

_{supercooled phase}

I.

Thus,

the

_phase

situation in solid

_cyclohexanol

as

_{given by}

Green and Griffith is

_quite

different from that

reported by

Adachi

_{et al.,}

_apart

from the

_temperatures

of

_{respective phase}

_transitions,

which

are in

_fairly

_good

_agreement.

The essential difference between the two

_{reported phase}

diagrams

of

_cyclohexanol

is seen most

_clearly

in

_figure

_1,

where relevant free

_enthalpy

schemes are sketched.

Fig.

1. - Phase

diagràms

of

_{cyclohexanol postulated by}

Green

_{[3] (left)}

and Adachi

_{[2] (right).}

The far infrared

_spectra

of solid

_cyclohexanol

studied

_{by gciesiiiska}

and

Sciesinski

_[4]

showed that

_phase

III indeed transforms into

_phase

II at a certain

_temperature

_range

_(e.g.

at

238 K

_during

ca 2

h)

but the authors did not find that transition to be reversible. The authors also found that the

_{supercooled phase}

1 transforms at ca 200 K into

_phase

III via an

intermediate metastable

_phase

_MS,

which

_{probably corresponds}

to

_{phase pre-II}

in

_[3].

The

literature data

_quoted

above as well as _{the later papers}

_[5-9]

do not

_give

a definitive

explanation

of the nature of the

_particular

solid

_phases

of

_{cyclohexanol.}

Phase 1

_(with

the

symmetry

Fm3m

_[7])

is

_undoubtedly

a so-called

_plastic

or ODIC

_{(orientationally disordered)}

phase.

This conclusion results from the

_broadening

of the

QNS

_peak

_[8],

the low value of

melting

entropy

(in

agreement

with

Timmermans criterion

_[11])

and the values of e’ and E"

_[12].

However,

the nature of this

_plasticity

remains unclear.

_Suggestions

from IR

(1)

In the

_original

_{paper of Green and Griffith that}

_phase

was denoted

_phase

III and the

measurements

_conceming

the

_ordering

of molecules in

_phases

_II,

III and MS

_{[4, 13]}

need

confirmation

_by,

first of

_all,

_good

diffraction measurement. Some new results obtained

_by

means of neutron

_scattering

were

_{published recently}

_[10].

The main aim of the

_present

_{paper is} to solve the

_controversy

_conceming

the

ther-modynamic stability

of

_{particular phases. Specific}

heat measurements were

_performed

_mainly

in order to observe the

_phase

III to

_phase

II transition and decide whether these

_phases

form a

monotropic

[2]

or an

_{enantiotropic}

_system

as

_suggested

in

_[3].

In addition we studied the

reversibility

of

_phase

transitions in solid

_{cyclohexanol, taking advantage}

of the distinct

differences of far infrared

_spectra

for

_{particular phases. Generally,}

we

_applied

the adiabatic

calorimetry

and IR

_spectroscopy

as

_{complementary}

methods to

_get

more information on the

polymorphism

of the

_system

studied.

2.

_{Experimental.}

The

_specific

heat of

_cyclohexanol

was measured in the

_temperature

_{range of 170-320 K}

_by

means of the

_{low-temperature}

automatized adiabatic calorimeter with

_platinum

thermometer

[14].

In the

_specific

heat runs the standard amount of heat

_supplied

resulted in a

_temperature

rise of the

_sample

of ca 2.5 K. The AC

_bridge

used for the thermometer resistance measurements _{gave the}

_sensitivity

of the

_temperature

measurement better than

10- 5

K. The time

_required

to achieve thermal

_equilibrium

was ca 2 h and in the

_vicinity

of

_phase

transitions

it became much

_longer.

The

_enthalpy

of

_phase

transitions was determined in

_separate

runs.

Apart

from the classical adiabatic _measurement, the calorimeter was also used in a

_dynamic

mode. In that mode a constant heat flow from or to the

_sample

was induced

_by

_proper

adjustment

of the adiabatic shield. Then the

_temperature

of the

_sample

as a function of time

was recorded. Commercial

_cyclohexanol

_fractionally

distilled was used in the

_experiment.

The mass of the

_sample

was _{57.72 g, which is} 0.5763 mol. The total volume of the

_sample

holder

was ca 80

cm3.

Far infrared

_spectra

were measured

_using

the Fourier Transform

_{Spectrometer Digilab}

FTS-14 in the same conditions as described in

_[4].

The measuréments were

_performed

in the

frequency

range 100-500 cm-1 with the resolution 2 cm-l. The

_sample

_temperature

was

stabilized with an _{accuracy of} ± 0.5 K. A

_temperature

_programmer was used in the

_growing

of

particular phases.

3. Results.

Figure

2 shows our

_experimental

values of the molar

_specific

heat of

_cyclohexanol

_(2).

Smooth

lines described

_by

3-rd order

_polynomials

_(W(T)

=

Ao

+

_{A, *}

T +

_A2

*

T2 + A3

*

T3)

were

drawn for

_{particular phases.}

The coefficients of the

_polynomials

were obtained

_by

a least

squares fit to the

_{experimental points lying}

outside the anomalies connected with the

_phase

transitions. These coefficients are collected in table I. The

_specific

heat values found in this

paper for all

phases

are

_higher

than those

_reported

in

_[2]

_by

1.8 % on the average. The

thermodynamic

parameters

of the

_phase

transitions studied are

_given

in table

_II,

where the

relevant values

_{reported by}

Adachi et al.

_[2]

are included for

_comparison.

The

_enthalpy

and

entropy

changes

for the transitions were calculated

_assuming

the transitions to be isothermal at the transition

_{temperatures.}

The III --> II transformation is

formally

treated here as if it were a reversible transition with the

_crossing

of free

_energies

of

_phases

III and II. We discuss

this

_point

in section 4 and conclude that this is not the case.

Fig.

2. - The molar heat

capacities

of

cyclohexanol.

Table 1. -

Coefficients

of

the

_polynomials

_{for different}

_phases.

Table II. -

Thermodynamic

parameters

of

the

_phase

transitions studied.

*

The thermal treatment of the

_{sample by}

which different

_phases

of

_cyclohexanol

were

obtained was based on the

_experience

of

_[4]

and is

_{schematically}

shown in

_figure

3.

Fig.

3. -

Diagram

of the

applied

thermal treatment of the

sample.

3.1 MELTING. - The

_phase

1 to

_liquid

_phase

transition for the

_{cyclohexanol sample}

studied in this paper took

place

at the

_temperature

_T.

= 297.92 ± 0.02 K. From the fractional

melting

study

(assuming

no formation of a solid

solution)

the

_melting

_temperature

of pure

cyclohexanol

was estimated to be

_Tm

= 298.27 K and the

degree

of

impurity

of _our

sample

_to

be of the order of 0.09 mol %.

_However,

our

_Tm

is lower than the value

_{given by}

Adachi

_by

0.76

K,

which indicates that the

_degree

of

_impurity

of our

_sample

must be

_higher

than estimated.

_During

the

_cooling

of the

_liquid

with constant heat flow from the

_sample

the minimum

_cooling

rate associated with

_freezing

was found at T - 293

K,

which indicates that a

hysteresis

of the

_phase

1 --->-

liquid

transition is

significant.

3.2 PHASE III. - The initial

procedure

was to _grow

_phase

III from the

_{supercooled phase}

1 at

a

_temperature

of ca 200 K. The

_sample

was cooled from room

_temperature

to that

_point

and then the

_côoling

was

_stopped.

The

_cooling

rates used in far infrared measurements varied from 1 K/min to 6 K/min. At ca 200 K the

_sample

transferred

_{spontaneously during}

several hours into

_phase

III via an intermediate metastable

_phase

MS.

_Figure

4 shows an

_example

of time evolution of the FIR

_spectra

at the

_temperature

of 198 K. The

_spectra

_correspond

respectively

to _{the initial pure}

_phase

_I,

mixture of

_phase

1 and

MS,

mixture of MS and III and

the

_resulting

_pure

_phase

III. It was found that the rates of transitions :

_phase

1 --->- MS and

MS -

_phase

III

_depend

_quite

_strongly

on the

_temperature

at which the

_cooling

of

_phase

1 was

stopped.

The

_{corresponding}

rates are the

_highest

at ca 198 K and 213 K and are indicated in

figure

3

_by

different thickness of lines.

The fact that the

_phase

1 to

_phase

III transition takes

_place

in two

_steps

was also seen in the

calorimetric measurements. The

_dependence

of the rate of the

_sample

_temperature

_change

vs.

time,

i.e.

_{dT/dt (t )}

at constant heat flow from the

_sample,

exhibits two maxima

_(see

_Fig.

_5).

These maxima are caused

_by

the release of

_enthalpy

from the

_sample

and

_they

indicate that

both transitions are exothermic.

Specific

heat measurements for

_phase

III were

_{performed only}

_up to ca 220 K because

above that

_temperature

_phase

III was

_loosing

its

_stability.

It was

_transforming

to

_phase

II. This fact was observed

_previously

in FIR

(see

_Fig.

3 in

[4])

and IR measurements

_(Fig.

2 in

[13]).

This III ---> II transformation on increase of

_temperature

was

_recently

observed in the

thermal

_conductivity

measurement

_[9].

In our calorimetric measurements the

_instability

of

Fig.

4. - Time evolution of FIR

spectra

illustrating

the two _step

_phase

transformation from

_supercooled

phase

1 to III via MS

_phase

_(at

198

_K).

Fig.

5. -

Spontaneous

heating

effects

_illustrating

the two _step

_phase

transformation from

_supercooled

phase

I to III via MS

_phase.

energy

input

above 220 K. After

attaining

a

_{temporary apparent}

_equilibrium

in the normal

time of 2

h,

the

_sample

_temperature

started to

_decrease,

_although

the

_sample

was

_kept

in

adiabatic conditions.

_Figure

6

_presents

an

_example

of this

_spontaneous

_cooling

of the

_sample

during

the endothermic _{process. The} duration _{of this process became much}

_longer

on

lowering

the initial

_temperature

of the

_sample

_(e.g.

ca 80 h for

_T;nitial

= 222

K):

Therefore,

in

order to

_study

the

_phase

III to II transformation in the

_vicinity

of 220 K an

_experimental

series

was

_performed

in which

_subsequent

amounts of heat

_{(Q -}

78

_{J )}

were

_supplied

_{every 24}

_h,

although

this

_long

time was still too short to obtain

_good

thermal

_equilibrium.

_Figure

7

Fig.

6. - Abnormal

cooling

effect due to the endothermic

_phase

III to II transformation observed in thermal drift

_{dT/dt (continuous}

line and filled

_dots)

and _temperature

_{(empty dots).}

Fig.

7. - Results of the

non-equilibrium

measurement of the _apparent heat

_capacity

in the range of

phase

III to II transformation.

to calculate the

_{enthalpy àHIII _>}

II needed to transform

phase

III to

phase

II

isothermally

at

the

_temperature

of the maximum of the curve in

_figure

7 i.e. at

_TmaX

= 220.93 K. The

enthalpy

value found in this way is AH = 378 J/mol.

However,

the results

presented

do _not

enable us to determine whether the transition is reversible

_{(enantiotropic}

_system

of

_{phases’ III,}

and

_II)

or is not

_(monotropic

set of

_phases).

An

_attempt

to observe the inverse

transition,

i.e. II -

III,

in

spectroscopic

measurements was a failure. To

_provoke

this

transition,

a mixture of

phases

III and II was cooled to various

_temperatures

below 220 K and FIR

spectra

were

recorded. No

_changes

in the

_spectra

were observed in reasonable time

(several hours).

This result may indicate an absence of

_{reversibility}

but does not exclude

_it,

because the

_phase

II to III transition _may

_be,

for

_{example, strongly}

hindered

_by

kinetic factors.

As Adachi et al. did not observe the

_phase

III to

_phase

II transition but claimed that

_phase

III transforms at 244.8 K

directly

into

_phase

_I,

we also tried to observe that transition. In

spectroscopic

measurements

_phase

III was heated from a

_temperature

lower than 220 K to

248 K with the

_heating

rate of 6 K/min and each time the

spectrum

recorded at 248 K indeed

was the

_spectrum

of

_phase

I. It turns out,

then,

that with a

_heating

rate

_{high enough}

the

sample

can

_jump

over the

temperature

range where it transforms into

phase

II. With our

calorimeter such a

_{high heating}

rate was not feasible.

_However,

it was found that

_phase

III may be obtained from the

supercooled

phase

1 also in another

_temperature

_{range, different} from that stated at the

_beginning

of this section. That is to _{say, it} can be grown at a

temperature

a little over 220 K.

_{Spectroscopic}

measurements confirmed that the

_phase

obtained _{in this way indeed is}

_phase

III and grows without an intermediate

_phase

MS

(see

Fig.

8).

The

_phase

_{III grown in this way}

_probably

contains traces of

_phase

1 but no nuclei of

phase

II and it does not _{exhibit any}

_tendency

to transform into

_phase

II. Thanks to that the calorimetric measurement could also be

_performed

in the

_vicinity

of the

_phase

III to

_phase

1 transition. The transition

_temperature

_obtained,

_{TIll -+}

₌ 244.5 ± 0.1

_K,

is in

_good

_agreement

with the value

_{given by}

Adachi et al.

_[2].

After the III --> 1 transition we did not observe the

« sluggish

exothermal transformation from

_crystal

1 to

_crystal

II »

_reported

in

_[2]

and

_[5],

because our

_phase

I obtained at the III - 1 transition was not contaminated

_{by phase}

II nuclei. Under

_{subsequent cooling}

and

_annealing

at a lower

temperature

(e.g.

at 170

_K)

_phase

III _{obtained in this second way would} on slow

_heating

tend to transfer to

_phase

II

_again.

A detailed

_analysis

would

_require

a

_separate

and more

_{systematic study.}

_{Nevertheless,}

a

_large

separation

in the

temperature

scale of the nucleation and

_growth

_{processes of}

_phase

II is clear.

3.3 PHASE II. - Phase II

was

_always

obtained via the

_phase

III to

_phase

II transition

described above. Next it was cooled to 170 K and the routine

_specific

heat runs were

performed.

In the cases when the

_newly

obtained

_phase

II was not cooled

_prior

to the

measurement, the heat

capacity

values were

_slightly

lower

(on

_average

_by

ca 0.1

%).

This may

indicate

that,

after

_being

obtained from

_phase

_III,

_phase

II at first contains traces of

_phase

III,

Fig.

8. - Time evolution of FIR

spectra

illustrating

the direct transformation from

_{supercooled phase}

1

which

_disappear

on

_cooling.

It was found in calorimetric measurements that

_phase

II transforms into

_phase

1 at

_{TII _>}

_{I =} 264.86 K + 0.02 K and no indication of

instability

of this

phase

was observed in the

_temperature

_{range studied. As}

_phase

II was _{grown from}

_phase

III

and not

_directly

from

_phase

_I,

we decided to

_study

the

_{reversibility}

of the II -> 1 transition

using

the

_{spectroscopic}

method. Phase II was

_partly

transformed into 1 at 266 K and next the

mixture was

_quickly

cooled

_{(6 K/min)}

to several

_degrees

below

_{TII->.I.}

The time evolution of FIR

spectra

was then studied.

_Figure

9

presents

an

_example

of the evolution of the

spectra

for

a mixture

_containing

_{almost pure}

_phase

1

_transforming

into II at 253 K. This result shows that the II ---> 1 transition is reversible.

Nevertheless,

all _our varied

_attempts

to

_get

a similar result

starting

from pure

phase

1

_(not

_containing

nuclei of

_phase

_II)

failed. This means that the

supercooled phase

1 has

_{good stability}

in this

_temperature

_region.

Thanks to this it was

possible

to measure

_specific

heat for

_phase

1

_beginning

at a

_temperature

of ca 240 K. Below

this

_temperature

_phase

I transformed into

_phase

III.

Fig.

9. - FIR

evidence of the

_{reversibility}

of

_phase

II to I _{transition ;} the _{spectrum of pure}

_phase

I

(dashed line)

is

_given

for

_comparison.

4. Discussion.

In this section we discuss a

_{problem already}

mentioned in the

_{introduction,}

_namely,

whether

phases

III and II form an

enantiotropic

or a

_monotropic

_system.

_Basing

on our results for the

specific

heat and

_{thermodynamic}

_parameters

of the

_phase

transitions we

_performed

an

enthalpy

and

_entropy

balance

_starting

from a chosen

_equilibrium

_temperature

in

_phase

III

(e.g.

220

_K)

and

_ending

at a

_given

_temperature

in

_phase

I

_(e.g.

270

_K).

The

_enthalpy

and

entropy

changes

cannot

_depend

on the way from the initial to the final

_{equilibrium points.}

We did the calculations for two routes,

namely

A :

_phase

III -->

phase

1 and B :

phase

III -->

phase

II -->

phase

1

assuming

that the III --> II

phase

transition is reversible and its

_parameters

_{are as}

given

in table II. It turns out that the

_entropy

_change

calculated on route A amounts to

63.83 ± 0.3 J/mol K and is

_by

2.5 J/mol K

_higher

than that calculated on route B. As the

observed transition

_temperature

_{TIII ...>. II}

_{may be}

_higher

than the actual one we

_repeated

the

calculations for lower

_{TIII...>. II}

_temperatures

but the

_entropy

balance still did not _agree.

involves the

_entropy

_production

which is not included in the calculations. This result solves the

_controversy

between the older Adachi et al.

_[2]

and Green and Griffith

_[3]

_{papers in} favour of the

_{monotropic phase}

II -

phase

III

_system

_postulated

in

_[2].

Phase III is then

metastable with

_respect

to

_phase

II in the whole

_temperature

_{range of} existence of

_phase

III.

This _{agrees with the fact that}

nobody

has

_reported

an observation of the inverse i.e. II --> III

transition.

The

_temperature

_dependence

of the reduced free

_enthalpy

based on our

_experimental

data is shown in

_figure

10. As our measurements were not

_performed

to low

_enough

_temperatures

the

_point

for

_phase

1 at 290 K was taken from

_[2].

The MS

_phase

is not included in

_figure

10

because it is not

_possible

to make adiabatic calorimetric measurements for this

_phase

on

account of the fast irreversible MS - III transformation. One could

_{roughly place}

the free

enthalpy

line above that of

_phase

III with the reversible MS - 1 transition somewhere below

240 K. The direct MS - 1 transition was earlier observed

_by

the far infrared method under very fast

heating

conditions

_[4].

Fig.

10. -

Experimental

reduced free

_enthalpy

of

cyclohexanol

(the

fitting point

for

_phase

1 at 290 K from Adachi

_[2]).

5. Conclusions.

The

_monotropic

_system

of

_phases

II and III

_{postulated by}

Adachi et al. was confirmed. Phase

III in the whole range of its existence is metastable with

respect

to

_phase

II.

_Additionally

to

the

_{previously reported}

_{[4, 13]}

infrared observation of the irreversible

_phase

III to II

transformation the same _process was visualized

_{calorimetrically,}

_giving

some hints about the

kinetics of the transition. The calorimetric evidence of the transition from the

_supercooled

phase

1 at ca 200 K to

_phase

III via an intermediate MS

_phase

is

shown. An

interesting

feature

of

_phase

_{III grown}

_directly

from

_supercooled

1

_(without

the intermediate MS

_phase)

at

T > 220 K was noted :

_phase

_{III grown in this way exhibits much} more

experimental

stability

and does not _{show any}

_tendency

to transfer to

_phase

_{II up} to the III - 1 transition. A

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