Evidence of production of H2CN+-N2. Application to the atmosphere of Titan

HAL Id: jpa-00232082

https://hal.archives-ouvertes.fr/jpa-00232082

Submitted on 1 Jan 1982

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Evidence of production of H2CN+-N2. Application to

the atmosphere of Titan

C.V. Speller, M. Fitaire, A.-M. Pointu

To cite this version:

C.V. Speller, M. Fitaire, A.-M. Pointu. Evidence of production of H2CN+-N2. Application to the atmosphere of Titan. Journal de Physique Lettres, Edp sciences, 1982, 43 (14), pp.499-503. �10.1051/jphyslet:019820043014049900�. �jpa-00232082�

Evidence

of

_production

of

_H2CN+

_-N2.

Application

to

the

_atmosphere

of

Titan

_(*)

C. V.

_{Speller (**),}

M. Fitaire and A.-M. Pointu

Laboratoire de _Physique des Gaz et des Plasmas _(***), Université Paris-Sud, 91405 _Orsay Cedex,

France

(Re~u le 23 avril _{1982, accepte} le _{1-juin 1982)}

Résumé. 2014 La molécule

_H2CN+-N2

est mise en évidence dans un _{mélange N2-CH4} soumis au

rayonnement alpha d’une source radioactive. La constante _{d’équilibre} est mesurée et la valeur de

l’enthalpie de la réaction a été déterminée

(|

0394H0|

_{= 7,65} ± 0,09 kcal.mole-1

_).

Abstract. 2014 The molecule

H2CN+-N2

has been observed in a mixture of _N2-CH4 irradiated _by

alpha particles from a radioactive source. The _equilibrium constant has been measured and the

enthalpy of the reaction has been calculated

_(|

0394H0|

= 7.65 ± 0.09 kcal . mole-

_1).

Classification

Physics Abstracts 82.30 - 96.30M

1. Introduction. -

Titan,

the

_largest

satellite of

Saturn,

is one of the smallest

_objects

in the solar _system to have an

_atmosphere.

Evidence of

_complex

chemical

_phenomena

arising

in this

atmosphere

is of _great

interest,

both in

_{cosmochemistry}

and in

_exobiology.

This concerns

parti-cularly

chemical processes involved in the formation of

_prebiotic

_molecules,

_probably

similar

to those which could have acted in the

_primitive

earth’s

_atmosphere.

Infrared measurements

_{performed by}

VOYAGER I

show

that Titan’s

_atmosphere

is

_mainly

composed

of molecular

_nitrogen

_(~

90

_{%) [1]}

and methane

_{( ~}

5

_%).

Small amounts of

_{H 2,}

C2H2,

C2H4,

C2H6,

C3Hg

and

_{C4H2 produced by photolysis}

of

methane,

have been

detected,

and also nitriles :

HCN,

C2N2

and

_{HC3N [2],}

which are able

to

_{play a}

fundamental role in the

formation of other

_prebiotic

molecules.

Moreover,

it is reasonable to _{presume that} more

complex

organic compounds

are also _present in Titan’s

atmosphere,

such

as

CH3CN,

C2H3CN,

C2HSCN

and,

may

be,

nitriles of

higher

order

[3, 4].

~ ,

Probably three-body

association reactions are

mainly

involved in the

synthesis

of such nitriles

[3, 5]

in Titan’s lower

_{atmosphere :}

in

_fact,

_high

_{pressures and low temperatures} are

propitious

conditions,

while ionization results from cosmic rays

impinging

on the

_{atmosphere. Among}

all

the ions able to

_{play a}

role,

H 2CN +

is

_probably

of _great

_{importance :}

it is

assumed

to be

res-ponsible

for HCN formation

_[6],

while further more it can be combined

with

molecules

N ~,

(*) La version française de cet article a été _proposee pour publication aux _Comptes Rendus de 1’Academie des Sciences.

(**) Supported by C.N.Pq. and CAPES (Brazil). (***) Associated with the C.N.R.S.

L-500 JOURNAL DE _{PHYSIQUE -} LETTRES

CH4

and

_C2H2

to

_{produce compounds}

such as

CH3CN, C2H3CN,... through three-body

reactions.

Until now,

only

little information is known about chemical reactions

_involving

such ions.

Figure

1 illustrates some

possible

schemes

leading

to

_H2CN+

formation in Titan’s

_atmosphere

[3, 6].

In the

_following,

we shall _present results of

laboratory

measurements

_concerning

the for-mation of

_H2CN+-N2

from

_H2CN+.

Fig. 1. - Schematic

diagram of N + ion-molecule reactions with _CH4 and _{N2 leading} to _production of

H2CN+.

2.

_Experimental

device. - An

_experimental

set _{up has} been

_designed

to allow the observation of ions

_produced

in

variable,

relatively high

pressure and low temperature gas

( 1

to 700 _torr, 10 to 300

_K)

from low

_{activity alpha}

sources

(2 41

_Am,

40

IlCi/cm2).

Ions

resulting

from

_primary

ionization are

subjected

to collisions with neutral

_species

which lead to formation of several

secondary

ionic

_species.

These are extracted from the gas chamber

_through

a small hole

_(diameter

0 =

50

_~m),

then driven to a

_quadrupole

mass _spectrometer

_(MS)

_acting

as an ion

_analyser.

A _cryostat allows one to _{vary the gas temperature} which is measured

_using

an AsGa

_probe

situated in the _gas chamber. _{The neutral gas}

_composition

is determined

_by

means of a second

MS downstream from the

_extracting

hole.

3.

_Experimental

results and discussion. 2013

_Figure

2 shows an

_example

_of

a recorded _spectrum in the case

of a N2-CH4

_{( ~}

100 :

_{1 )}

mixture at a _{pressure p} = 40 torr and _{a temperature} T = 262 K.

Amplitudes

of

_m/e

= 28 and 56 observed

peaks

_are

mainly

due

respectively

_to

H2CN +

and

H2CN+ -N2.

This has been shown

_by

_{substituting CD4}

for

_CH4

in the same

_proportion

mixture : the two

_previous

_peaks

are then observed at

_m/e

= 30 and

58,

and

_nothing

remains at

_m/e

= 28

and 56 but low

_{amplitude peaks}

attributed to

_N;

and

_{N4 .}

The

_identity

of these last ions has been confirmed

_{similarly by}

_substituting

for mass 14

_nitrogen

an

_isotopic

mixture of

_{equal quantity}

of

mass 14 and mass 15

_{nitrogen :}

the

_peak

_{corresponding}

to

_mle

= 28 is then

_splitted

into two

_peaks

of

_{comparable amplitude}

at

_m/e

= 28 and

29 ;

the

peak

at

mle

= 56,

which

implies

three

nitrogen

atoms, then appears at

_m/e

=

56, 57,

58 and 59.

Such recorded

_spectra

allow one to determine the

_equilibrium

constant

_{[3] K 1 corresponding}

to the reaction

In the

_following,

we shall assume that reaction

₍₁₎

is reversible. This will be verified later.

_K,

is

Fig. 2. - Mass

spectrum in a mixture of

_N2-CH....

_Beyond

H2CN+

and

_H2CN+-N2

ions, the _probable chemical formula attributed for other detected ionic _species are indicated _(traces of _hydrogen and of hydro-carbons _(C2H6,

_C3Hø,

_...) contribute for the _production of some of the ionic _species). With MS _sensitivity chosen for spectrum clarity, the _amplitude of the

_H2CN+-N2

_peak is out of scale; this _peak is thus shown with a MS _sensitivity divided _by ten for _comparison with the other _peaks.

where brackets indicate densities of

_{corresponding compounds.}

In our

experiment,

the ratio of

ionic densities is

_equal

to the ratio of

_{peak amplitudes}

for masses 56 and

_28,

and

_{nitrogen density}

is

_{given by}

the _pressure measured in the _gas chamber.

Figure

3 shows variations of

_{Log K 1}

as a function of T

_(Van’t

Hoff

_plot).

These variations are

deduced from measurements at several

_nitrogen

pressures and several molar fraction of methane

as well in the case

_{of CD4}

or 15N.

One can observe a linear

variation,

independent

of

_nitrogen

_{pressure in} the

_region

studied

(20

to 80

_torr)

and

_independent

of the methane

_proportion

_(I

to 5

_%).

This indicates that a

thermo-dynamic equilibrium

is reached and that the

_{reversibility assumption}

is valid

Points

_{corresponding}

to measurements with

_CD4

deviate

_notably

from the drawn line. This is due to _{oxygen, present} in small

_proportion

_{in the gas,} which leads to formation of NO + ions with

L-502 JOURNAL DE

_{PHYSIQUE -}

LETTRES

Fig. 3. - Van’t Hoff

plot of the

_H2CN+

+ 2 _{N2 #}

_{H2CN+ -N2}

+ _N2 reaction for a _N2-CH4 mixture

(~ 100: _{1) :} x ; 20 torr; 0, 40 torr; +, 80 torr ; 0, measured in _{N2 (m} --fit ₁₄ and _{15)~CH~; CL measured}

in,N2-CD4..

’ ~

..

,

~

-

-... .

calculation

_{of K 1}

erroneous.

_{Complementary measurements}

_show

_that

the

_proportion

_of

_{NO +}

increases with

_decreasing

_temperatures and is

_negligible

at room _temperature. The

_point

obtained

for 300 K is thus the

_only

one to be valid.

"

Another

_point

can also be

_plotted

on the

_K

₁ curve, deduced from a recorded _spectrum

using

14 N

and’15N

mixture.

Nevertheless,

its

_precision

is

atso smaller

because it was _necessary to add the

amplitudes

of

the four

_peaks

of

ions

_H~CN~-N~

and

_{the two peaks}

of ions

_H2CN+

to

deduce

the

ionic

densities of

_interest.,

.

,..’,~, " ..

The standard

_enthatpy,

_A7~,

of reaction

₍₁₎

can be

estimated,

using

the curve

_{of K,}

as drawn

on

_figure

3,

and

the relation - .

’

’

, ,

R is the

_perfect

gases constant and it is assumed that the variation in _entropy,

_AS~,

is small in the range of studied temperatures

(200-300 K).

We

_obtain

_{0~° ~}

_{I -}

7.65 ±0.09 kcal.

mole-1

_(the

relative error is determined

_using

statistic deviation from Van’t Hoff

_plot):

Such a value of

OH°

K 1

value at 150 K can be deduced

_{by extrapolating}

results of

_figure

3. We obtain

_K

₁ = 2.95 _x

103 torr-1. If the value

_k+

=

10 - "

cm 6 . S - 1 is attributed

[3]

to the constant of the forward

reaction,

leading

to the formation of

_H2CN+-N2,

it is then

_possible

to

_calculate,

_using

our

_K,

_i

value,

the reaction constant,

k_,

for the reverse _{process. We} obtain

k I x 10

cm3 . s" ~ at

150 K.

Besides the results

_presented

_above,

_analysis

of _spectra obtained at low _temperature

(T ~

150

_K)

for pressures around 40 torr, suggest that the ion

_H2CN+

can be

_possibly

associated with several

nitrogen

molecules. Association of other ionic

_species

with

_N2

or

_CH4

is also evidenced in these

spectra. Ions of

_global

formula

_{C2Hj, C3Hj, ...}

have been detected and identified

_using,

as

pre-viously, isotopic

methane or

_nitrogen.

4. Conclusion. - Our results indicate that the reaction

_H2CN+

+ 2

_{N2 f:+}

_{H 2CN + -N 2}

+

_N2

is

_possible

and can lead to formation of the ion

_{H 2CN + -N 2}

in Titan’s

_atmosphere.

Equilibrium

constants of this reaction have been measured for _temperature

_ranging

from 200 to

300 K

_allowing

one to deduce the

_enthalpy

_(I

_{AH 0}

₁

= 7.65 _± 0.09

kcal. mole - 1).

Furthermore,

existence of other

_possible

association reaction of

_H2CN+

ion with

_N2

or with

_CH4

has been shown. Such results are in _agreement with some

_expectations

_pointed

out in a recent

_study

_[3,

_6].

References

[1] HANEL, R. _{A., et al.,} Science 212 ₍₁₉₈₁₎ 192.

[2] SAMUELSON, R. E., HANEL, R. A., KUNDE, V. G. and _MAGUIRE, W. C., Nature 292 ₍₁₉₈₁₎ 686.

[3] CAPONE L. A., PRASAD, S. S., HUNTRESS, W. T., WHITTEN, R. C., DUBACH, J. and SANTHANAM, K.,

Nature 293 ₍₁₉₈₁₎ 45.

[4] MOUREY, D. and _{RAULIN, F.,} Nouveaux _{développements} dans la connaissance du _Système Solaire,

Joumée d’Etudes de l’ATP de _{Planétologie,} I.N.A.G. _(CNRS) Paris _{(1981) p. 98.}

[5] ALLEN, M. and _YUNG, Y. L., 13 DPS _Meeting, Bull. Am. Astron. Soc., vol. 13/3 (1981) 702.

[6] CAPONE, L. A., DUBACH, J., WHITTEN, R. C., PRASAD, S. S. and SANTHANAM, K., Icarus 44 ₍₁₉₈₀₎ 72.

[7] KEBARLE, P., SEARLES, S. K., ZOLLA, A., SCARBOROUGH, J. and _{ARSHADI, M.,} J. Am. Chem. Soc. 89

(1967) 6393.

[8] CASTLEMAN Jr., A. W., HOLLAND, P. M., LINDSAY, D. _{M., and PETERSON,} K. I., J. Am. Chem. Soc. 100