On the formation of niacin (vitamin B3) and pyridine carboxylic acids in interstellar model ices

ON THE FORMATION OF NIACIN (VITAMIN B3) AND PYRIDINE CARBOXYLIC

ACIDS IN INTERSTELLAR MODEL ICES

*

Brandon M. McMurtry, Andrew M. Turner, Sean E. J. Saito, and Ralf I. Kaiser

*

W. M. Keck Research Laboratory in Astrochemistry, University of Hawaii at Manoa, Honolulu, Hawaii, HI 96822

Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii, HI 96822, US

1

ABSTRACT

The formation of pyridine carboxylic acids in interstellar ice grains was simulated by electron

5 5 2

exposures of binary pyridine (C H N)-carbon dioxide (CO ) ice mixtures at 10 K under

contamination-free ultrahigh vacuum conditions. Chemical processing of the pristine ice and

subsequent warm-up phase was monitored on line and in situ via Fourier transform infrared

spectroscopy to probe for the formation of new radiation induced species. In the infrared spectra

of the irradiated ice, bands assigned to nicotinic acid (niacin; vitamin B3; m-C

5

H

4

NCOOH)

) were

along with 2,3-, 2,5-, 3,4-, and 3,5-pyridine dicarboxylic acid (C N(COOH)

5

H

3

2

unambiguously identified along with the hydroxycarbonyl (HOCO) radical. Our study suggests

that the reactive pathway responsible for pyridine carboxylic acids formation involves a HOCO

intermediate, which forms through the reaction of suprathermal hydrogen ejected from pyridine

with carbon dioxide. The newly formed pyridnyl radical may then undergo radical-radical

recombination with a hydroxycarbonyl radical to form a pyridine carboxylic acid.

2

1

. INTRODUCTION

Nicotinic acid (m-C

5

H

4

NCOOH) – commonly known as vitamin B3 or niacin – along with

NCOOH) and

5 4

isonicotinic acid (p-C H NCOOH; Figure 1), have recently been identified in the carbonaceous

5 4

the other two monosubstituted pyridine carboxylic acids, picolinic acid (o-C H

chondrites Murchison [1,2] and Tagish Lake [3] at levels of typically 7 ppm. A follow-up study

by Smith, et al. [4] on CM2-type carbonaceous chondrites revealed the presence of three

monosubstituted pyridine carboxylic acid isomers at levels from 163 to 1377 ppb along with

three disubstituted pyridine carboxylic acids (2,5-, 3,4-, and 3,5-) (Figure 1). These findings

build upon the complex inventory of organic molecules detected within chondrites [5] including

(

polycyclic) aromatic hydrocarbons [6,7], nucleobases [8], sugars [9,10], and amino acids [11-

1

3

12

15

14

3]. Detailed C/ C, N/ N, and D/H isotopic analysis explicitly indicated an interstellar

1

origin of these biorelevant molecules [1,2,7]. Among these complex organic molecules, nicotinic

acid (m-C NCOOH) in particular has received considerable attention due to its crucial role in

5

H

4

biological systems by serving as an important precursor to the redox coenzymes nicotinamide

adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) - key

components to cellular metabolic reactions [4]. Within the scope of modern metabolisms, NAD

and NADP, in coordination with enzymes, promote the replication and repair of

deoxyribonucleic acid (DNA), the ligation of ribonucleic acid (RNA), and cell differentiation

through transfer of its nucleotidyl moiety to nucleic acids and proteins [14]. NAD has also been

proposed as a potential catalyst to protometabolism on prebiotic Earth and to the origin of RNA

[

15,16]. These diverse implications of NAD in the prebiotic formation of complex, biorelevant

molecules emphasize the importance of nicotinic acid in early Earth environments.

Despite the importance of nicotinic acid (m-C NCOOH) in astrobiology and its role in the

5

H

4

Origins of Life theme, little is known on the underlying formation pathways in extraterrestrial

environments [14]. Conducting a retrosynthesis [17], the nicotinic acid molecule can be formally

decomposed into a pyridine molecule (C

5

H

5

N) plus carbon dioxide (CO

2

). Since both molecules

1

represent closed shell molecules in their A

1

and Σ

+

g

1

electronic ground states, the reaction of

pyridine with carbon dioxide to form nicotinic acid in the gas phase is affiliated with a

significant entrance barrier [18,19], which cannot be overcome in cold molecular clouds holding

averaged translational temperatures of typically 10 K. Further, even if formed in the gas phase

via the reaction of pyridine with carbon dioxide at elevated temperatures, a third body collider is

required to carry away the internal energy of the nicotinic acid prior to its unimolecular

3

-4

-5

-3

decomposition; however, due to the low number densities of typically 10 to 10 cm in old

molecular clouds, only bimolecular reactions take place. As a consequence, formation of

nicotinic acid in the gas phase of the interstellar medium is problematic. On the other hand,

complex organic molecules and carboxylic acids (RCOOH) in particular have been shown to be

formed in laboratory experiments in interstellar analog ices upon exposure of these model ices to

ionizing radiation such as ultraviolet photons, galactic cosmic rays, and energetic electrons

simulating the secondary electrons generated via the interaction of galactic cosmic ray particles

with interstellar ices [20,21]. More specifically, formic acid (HCOOH) [22], acetic acid

(

CH

3

COOH) [17], and complex organic carboxylic acids with ‘R’ being an organic alkyl group

RCOOH) [21] were formed in water-carbon monoxide, methane-carbon dioxide, and carbon

dioxide-hydrocarbon ices, respectively, upon exposure to energetic electrons. The initial step in

the reaction was found to be the cleavage of the oxygen-hydrogen (reaction (1)) or carbon-

hydrogen (reaction (2) and (3)) bonds upon decomposition of water, methane, and a generic

hydrocarbon, respectively. The released hydrogen atoms have excess kinetic energy of a few

electron volts (eV) and can overcome the barrier of addition to carbon monoxide (CO) of 0.12

2

eV and to carbon dioxide (CO ) of 1.10 eV forming the formyl (HCO) and trans-

hydroxycarbonyl (HOCO) radical, respectively (reactions (4) and (5)). If the geometry is

favorable, barrier-less radical – radical recombinations lead to the formation of formic acid (6),

acetic acid (7), and alkylcarboxylic acids (8) at 10 K within the ices.

(

1)

2)

3)

4)

5)

6)

7)

8)

H

2

O → HO + H

CH → CH + H

4

3

RH → R + H

H + CO → HCO

H + CO

HCO + OH → HCOOH

HOCO + CH → CH COOH

HOCO + R → RCOOH

2

→ HOCO

3

In analogy to these processes, the pyridine molecule can fragment upon exposure to ionizing

radiation via carbon-hydrogen loss to o-, m-, and p-pyridinyl radicals plus atomic hydrogen

(

reaction (9)), followed by reaction (5) with the suprathermal hydrogen atom, and recombination

of the pyridinyl with the hydroxycarbonyl radical (reaction (10)). Although not yet identified in

4

interstellar ices, pyridine may be formed in cold molecular clouds via the reaction of the cyano

5

4 6

radical (CN) with 1,3-butadiene (C H ) [23,24]. During the lifetime of a molecular cloud of 10

6

to 10 years, pyridine could condense on the interstellar grains at concentrations below the

detection limit of a few percent.

(

9)

C

5

H

5

N → o,m,p-C

5 4

H N + H

10)

o,m,p-C

5

H

4

N + HOCO → o,m,p-C

5 4

H NCOOH

Smith, et al. [25] attempted to emulate these conditions in laboratory experiments by exposing a

series of pyridine-carbon dioxide and pyridine-carbon dioxide-water ices in a high vacuum

-

7

chamber with pressures of a few 10 torr to energetic protons resulting in the formation of

pyridine mono- (o-, m-, p-) and disubstituted (2,3-, 3,4-, and 3,5-) carboxylic acids as identified

ex situ through mass spectroscopy utilizing a direct analysis in real time (DART-MS) ion source

and liquid chromatography coupled with UV detection and orbitrap mass spectrometry (LC-

UV/MS). However, the off-line detection of the carboxylic acids raises the question if these

molecules were formed within interstellar model ices and/or as a result of the hydrolysis of the

extracted residues at room temperature by the solvent (water-methanol) thus making it

impossible to define the source of the pyridine carboxylic acids, i.e. do these acids originate from

their monomers or do they resemble hydrolysis products from the polymeric residue.

Here we explore the formation of pyridine carboxylic acids in interstellar ices analogs

following irradiation with energetic electrons at 10 K on line and in situ. In contrast to previous

studies, our experiments were conducted under contamination-free ultrahigh vacuum conditions

-11

with pressures of 10 torr; further, the data collection was carried out on line and in situ thus

preventing contamination and hydrolysis of any residues. Furthermore, the on line studies

enabled us to conduct mechanistical studies on the formation mechanism(s) of these carboxylic

acids along with corresponding rate constants.

2

. EXPERIMENTAL

The experiments were carried out in a contamination-free, ultrahigh vacuum (UHV) chamber

-11

evacuated to a pressure of 5 × 10 Torr [26,27]. Desired ultrahigh vacuum conditions were

achieved through the utilization of a magnetically suspended turbo-pump (Osaka TG420MCAB)

backed by an oil-free scroll pump (Anest Iwata ISP-500). A highly polished silver wafer attached

to an oxygen free high conductivity (OFHC) copper target interfaced to a two-stage closed-cycle

5

helium refrigerator and programmable temperature controller capable of regulating temperatures

between 10 and 330 K was placed at the center of the chamber [28]. The premixed gas mixture

of pyridine-carbon dioxide was prepared by mixing 10 mbar of pyridine vapor (C

5 5

H N, Sigma

Aldrich, 99.8+ %) and 90 mbar carbon dioxide (CO , Airgas, 99.999 %) into an evacuated gas

2

mixing chamber. The pyridine used for this experiment was purified by triply freezing with

liquid nitrogen and evacuating the headspace, thus removing gaseous impurities such as nitrogen

and oxygen. The deposition of the ice was controlled by leaking the gas mixture through a

-

8

precision leak value and glass capillary array for 25 minutes at 10 K at a pressure of 5 × 10 torr

into the main chamber. The thickness of the ice was determined to be 540 ± 80 nm. Here, the ice

thickness was determined in situ through via laser interferometry [29,30] with two helium-neon

(

HeNe) lasers operating at 632.8 nm. Computation of the thickness also required a value for the

index of refraction for the ice mixture, to which the value for carbon dioxide (ꢀ_ꢁ_ꢂ_ꢃ= 1.245 [31])

was used based on the approximation that the ice composition closely resembled that of a

pristine carbon dioxide ice. Relative abundance of the two starting materials was determined via

a modified Beer-Lambert Law [26]. For carbon dioxide, the average column density was

1

of CO

3

determined based on the integrated areas of ν

1

+ν

3

and 2ν

2

+ν

3

of CO

2

and ν

3

2

, along with

-18

-1

-19

their corresponding absorption coefficients of 1.4 × 10 cm molecule , 4.5 × 10 cm

-1

-17

-1

molecule , and 7.8 × 10 cm molecule , respectively [32]. From this, the average column

1

7

-2

density of carbon dioxide in the ice mixture is found to be (2.64 ± 0.08) × 10 molecule cm .

-

3

After taking into account the solid density of carbon dioxide (0.98 g cm [33]) the average

thickness of the carbon dioxide component is determined to be 480 ± 30 nm, translating to a

2 5 5

relative abundance of approximately 8:1 (CO / C H N).

Figure 2 depicts the infrared spectrum of the deposited ice prior to the irradiation recorded

with a Nicolet 6700 FTIR Spectrometer (reflection angle α = 75° [20]) along with vibrational

assignments based upon literature values [35,36]. The assigned bands allude to the deposition of

a composite ice film, with vibrations corresponding to pyridine and carbon dioxide present. Most

1 3 3 2

notably, the ν +ν , ν , and ν absorptions of carbon dioxide, present at 3750-3690, 2500-2300,

-

1

and 700-650 cm , respectively, correspond closely to interstellar model ices studied by Bennett,

et al. [36]. Likewise, evidence for C-H stretching and ring modes of pyridine are apparent in the

-

1

3

100-2950 and 1370-1000 cm regions, respectively, aligning with literature values of solid-

state pyridine [35] (Table 1). The ice was then irradiated for 60 minutes with 5 keV electrons at a

6

current of 100 nA from a Specs EQ 22-35 electron gun. The electron beam exposed an area of

2

3

.2 ± 0.3 cm at an angle of 15° relative to the surface normal with an actual extraction

efficiency of 78.8 % of the electrons by scanning the beam over the ice surface. The electron

trajectories and energy transfer inside the ices was modeled by the CASINO program [37]. These

simulations yielded an imparted energy of 4.7 ± 0.5 keV per electron at an average penetration

depth of 390 ± 20 nm (less than the thickness of the ice to avoid interaction of the electrons with

the silver wafer) translating to an average of 4.0± 0.1 eV absorbed per molecule in the deposited

-

1

ice. This value corresponds to an average linear energy transfer (LET) of 12 keV µm , on the

order of magnitude 10-20 MeV cosmic rays transfer to interstellar ices ([38,39]). The irradiated

ice remained at 10 K for an additional 60 minutes before being heated to 293 K at a rate of 0.5 K

-

1

minute . In situ FTIR data were collected throughout the irradiation and temperature

programmed desorption (TPD) protocol. Mass spectra (Balzers QMG 422 Quadrupole Mass

Spectrometer) were collected up to a mass-to-charge ratio of m/z = 200 using an electron impact

ionization energy of 100 eV at an emission current of 0.2 mA to monitor gaseous species

released during the TPD phase [40,41].

3

. RESULTS

. 1. Infrared Spectroscopy

. 1. 1. Qualitative Analysis

Evidence for the radiation-induced formation of new species in the binary ices is apparent by

novel bands present in the post-irradiation ice mixture at 10 K. Due to the newly formed

carboxylic acid species not desorbing during the TPD phase, QMS data failed to indicate the

formation of these species. However, volatile reactants and light carbon species – such as carbon

2

monoxide (CO) and carbon dioxide (CO ) – were monitored to determine the temperature of

desorption. The absorption features were deconvoluted exploiting Gaussian fits (Figure 3)

utilizing the peak fitting application on GRAMS-AI. Details on the algorithm employed by

GRAMS-AI for fittings are described in Press, et al. [42]. Peaks were iteratively fit to spectra so

that the residual of the unfit portion was equivalent to the baseline noise. In situ spectra

throughout the warm-up period are presented to display bands resulting from radiation products

present following the sublimation of the volatile reactants. Particular interest is directed to those

spectral regions associated with peaks characteristic to pyridine carboxylic acids and the

7

proposed reactive intermediate, the HOCO radical. The novel absorption features along with the

literature assignments are summarized in Table 2. A comprehensive summary of literature values

for all pyridine carboxylic acid species along with the corresponding bands observed in this

study is provided in Table 3 and Table 4.

The newly emerging bands found within the electron-irradiated ice are indicative of the

formation of a variety of pyridine carboxylic acids along with the hydroxycarbonyl (HOCO)

-1

-

3 3

radical, carbon trioxide (CO ), carbon monoxide (CO) at 2141 cm , and ozone (O ) at 1032 cm

1

-1

.

We were able to identify the HOCO radical based on the absorption at 1845 cm

2

corresponding to the ν (CO) stretching mode. These assignments are in agreement with previous

-

1

studies of the HOCO radical in an argon matrix at 1846 cm [43] and as a byproduct of electron

irradiation of interstellar model ices composed of carbon dioxide (CO )-hydrocarbon (C

2

n 2n+2

H ;

-

1

n= 1-6) and methanol (CH

3

OH)-carbon monoxide (CO) both at 1852 cm [21,41].

The processed ice at 10 K also displayed unambiguous evidence for the formation of the

-

1

biologically relevant nicotinic acid (m-C

5

H

4

NCOOH) based on a peak at 1555 cm (ν

8

[ring

stretch]). This observed band agrees closely with a study completed on nicotinic acid in

-

1

potassium bromide (KBr) pellets, both of which assigned ν

8

at 1540 cm [44]. Additionally, 2,5-

,

3,4-, and 3,5-pyridine dicarboxylic acid (C H N(COOH)

5

3

2

) were identified unambiguously

-1

based on their bands at 1775 cm (ν(C=O)), 1638 cm (ν(ring)), and 1359 cm (ν33 [ring

stretch]), respectively. The 2,5- and 3,4- were assigned to their respective carriers on the basis of

a study conducted on the full suite of solid pyridine dicarboxylic acids in KBr. The two

aforementioned vibrations are unique to the 2,5- and 3,4- isomers, exhibiting literatures bands at

-

1

-1

770 cm and 1640 cm [45], respectively. Likewise, Nataraj, et al. [46] in the study of 3,5-

1

-

1

pyridine dicarboxylic acid in KBr found ν33 to occur at 1357 cm , matching the peak observed

within this study.

In our experiments we also observed the infrared spectra of the ices throughout the heating

-

1

-1

phase. In the irradiated ice at 129 K we observe new bands at 1811 cm and 1290 cm . From a

previously mentioned study on pyridine dicarboxylic acids [45] we find that the band at 1811

-

1

cm can be unambiguously assigned to the carrier 2,5-pyridine dicarboxylic acid due to its close

-

1

resemblance with the literature value of 1790 cm . The same study confirms the peak at 1290

-

1

-1

cm as σ(OH) of 2,3- pyridine dicarboxylic acid based on the published value of 1280 cm .

8

3

. 1. 2. Quantitative Analysis

Utilizing the peak assignments from section 3.1.1, a decrease of the column densities of the

reactants, pyridine and carbon dioxide, can be computed along with the yields of the radiation

products (Figure 4). For carbon dioxide, the temporal evolution of the ν +ν and 2ν +ν of CO

1

3

2

3

2

1

3

and ν

3 2

of CO were monitored to determine the quantity of carbon dioxide molecules

1

6

-2

destroyed. We determined that (3.4 ± 0.9) × 10 molecule cm carbon dioxide were degraded

-18

-1

-19

exploiting integrated absorption coefficients of 1.4 × 10 cm molecule , 4.5 × 10 cm

-

1

-17

-1

13

1 3 2 3 2 3 2

+ν and 2ν +ν of CO and ν of CO ,

molecule , and 7.8 × 10 cm molecule for ν

respectively. With the diminishing column density of carbon dioxide, new absorption features

corresponding to carbon monoxide and carbon trioxide were observed. Formation of the two new

carbon oxide species was monitored based on the fundamental band of carbon monoxide, present

-

1

-1

at 2141 cm , and the ν

1

of carbon dioxide at 2044 cm . Bennett, et al. [26] defined the integrated

-17

-1

absorption coefficients of the two aforementioned bands as 1.1 × 10 cm molecule and 3.1 ×

-17

-1

0 cm molecule , respectively. We find that over the course of the irradiation, the total

1

5

-2

increase in column density of carbon monoxide was (9.3 ± 0.8) × 10 molecule cm , while the

1

4

-2

total increase of carbon trioxide was (6.2 ± 0.2) × 10 molecule cm . Additionally, marginal

-

1

yields of the HOCO radical were recognized by the band at 1845 cm corresponding to its ν

2

-17

-1

band. Based on its integrated absorption coefficient of 3.6 × 10 cm molecule [17], the

1

3

-2

column density of the HOCO radical in the irradiated ice was (7.3 ± 0.9) × 10 molecule cm .

The total column density of newly formed carbon dioxide-based products is therefore computed

1

6

-2

to be (1.0 ± 0.1) × 10 molecule cm , i.e. 29 ± 9 % of the carbon dioxide was transformed into

carbon trioxide, carbon monoxide, and the hydroxycarbonyl radical. We conclude that the

difference between the amount of carbon dioxide destroyed and the column density of these

newly formed light carbon species must be the column density of carbon dioxide molecules

16

-2

incorporated into pyridine carboxylic acids: (2.4 ± 0.9) × 10 molecule cm .

For pyridine, the change in column density is determined by monitoring the temporal

-1

4

development of its ν and ν11 bands at 755 cm and 711 cm , respectively, using integrated

-

19

-1

-18

-1

absorption coefficients of 8.3 × 10 cm molecule and 2.4 × 10 cm molecule . Based on the

initial column density of pyridine in the pristine ice and the change in peak area of the

1

6

-2

aforementioned bands, an average of (1.6 ± 0.2) × 10 molecule cm pyridine molecules were

destroyed over the course of the irradiation. Since no other carriers containing the pyridine

9

moiety were identified in the processed ice, it can be concluded that all the destroyed pyridine

molecules were converted to pyridine carboxylic acids. Therefore, we establish the upper limit of

1

6

-2

the pyridine carboxylic acid column density of (1.6 ± 0.2) × 10 molecule cm . This

corresponds to an overall conversion of 6 ± 2 % of the carbon dioxide reactant molecules to

pyridine carboxylic acids. Completion of the mass balance for the irradiated ice involves

determining the average number of carboxyl groups incorporated into each newly formed

pyridine carboxylic acid. Based on the proposed reaction mechanism outlined by reactions (5),

(

9), and (10), it is determined the average number of carboxyl groups is equal to the column

density of destroyed carbon dioxide divided by the upper limit of the carboxylic acid column

density. In this manner, we find that an average of about 1.5 carbon dioxide molecules were

consumed per pyridine carboxylic acid molecule formed, implying a near equal mixture of

mono- and disubstituted pyridine molecules in the processed ice. These findings are consistent

with the assignment of a suite of mono- and disubstituted pyridine carboxylic acid species within

the irradiated ice as outlined above.

4

. DISCUSSION

Having assigned the carriers of the newly formed molecules within the irradiated pyridine-

carbon dioxide ice, we attempt now to elucidate the reaction mechanism involved in the

production of the identified pyridine carboxylic acids. Similar to the mechanisms extracted in

previous studies of carboxylic acid formation in interstellar model ices [21,39], the initial step in

the formation of pyridine carboxylic acids in this study is the loss of a hydrogen atom, as

described in reaction (9), from pyridine. This reaction is essentially a carbon-hydrogen bond

-

1

cleavage and is highly endogenic by 439 kJ mol (4.55 eV) for the o-pyridinyl radical and by

-

1

4

68 kJ mol (4.85 eV) for the m- and p-pyridinyl radicals [53]. The hydrogen atoms produced

possess up to a few eV of kinetic energy [17,21]. Hydrogen atoms with sufficient kinetic energy

-

1

are then capable of overcoming the entrance barrier of 106 kJ mol (1.10 eV) required to form

the trans-carboxyl radical (HOCO (reaction (5)). These newly formed radicals undergo radical-

radical recombination without barrier with the pyridinyl radical to form a monosubstituted

pyridine carboxylic acid. In an analogous process, the monosubstituted carboxylic acids can lose

a second hydrogen atom from the pyridine ring via interaction with energetic electrons to form a

radical and an energetic hydrogen atom. These monosubstituted pyridine carboxylic acid radicals

can then recombine with a hydroxycarbonyl (HOCO) radical – if the proper recombination

1

0

geometry can be reached – to form a suite of disubstituted pyridine carboxylic acids (reaction

11)). As nicotinic acid (m-C NCOOH) is the only monosubstituted species unambiguously

(

5 4

H

assigned, we deduce it is the primary intermediate in the formation of the observed disubstituted

acids: 2,3-, 2,5-, 3,4-, and 3,5- (reaction (12)). Due to the facile conversion of the

hydroxycarbonyl (HOCO) radicals to carboxylic acids, the aforementioned reaction mechanism

can be formally streamlined to reactions (13-14) (Figure 5).

(

11)

12)

13)

14)

C

5

H

3

NCOOH + HOCO → C

NCOOH + HOCO → 2,3-, 2,5-, 3,4-, 3,5-C

N + CO → C NCOOH

NCOOH + CO → C N(COOH)

5 3 2

H N(COOH)

m-C

5

H

3

5 3 2

H N(COOH)

C

5

H

5

2

5 4

H

C

5

H

4

2

5

H

3

2

These reaction pathways were also verified by kinetically fitting the temporal evolution of the

column densities (Table 5, Figure 6). The temporal evolution of the observed pyridine carboxylic

acids can be modeled by stepwise pathway A → B → C:

^ꢎꢃ

^ꢎꢐ

(

15)

ꢄC H NꢇCOOHꢈ ꢊ = ꢌ ꢍ1 −

ꢑ^ꢏ^ꢎ^ꢐ^ꢋ

−

ꢑ^ꢏ^ꢎ^ꢃ^ꢋ

ꢒ

ꢅ

ꢆ

ꢉ ꢋ

ꢎ_ꢃꢏꢎ_ꢐ

where ꢓ is the reaction rate of reaction (13) and ꢓ is the reaction rate of reaction (14). For this,

ꢔ

ꢉ

the bands assigned to 2,5- and 3,4-pyridine carboxylic acids were monitored throughout the ice

-4

-

irradiation. Figure 6 demonstrates the best fit of the two species where ꢓ = (4.3 ± 2.0) × 10 s

ꢔ

1

-4 -1

and ꢓ = (2.7 ± 1.2) × 10 s for 2,5-pyridine carboxylic acid and ꢓ = (4.5 ± 1.7) × 10 s

ꢉ

ꢔ

-4

-1

and ꢓ = (2.8 ± 1.0) × 10 s for 3,4-pyridine carboxylic acid. Collectively, we find that the

ꢉ

addition of a single carbon dioxide molecule to pyridine proceeds forward with a rate constant of

-

4

-1

(

4.4 ± 2.6) × 10 s , while the addition of a second carbon dioxide molecule to a mono-

-4

-1

substituted species proceeds with a lower rate constant of (2.7 ± 1.6) × 10 s . The highly

electronegative –COOH group likely reduces the rate of addition of a second hydroxycarbonyl

radical.

As described in section 3.1.2, the temporal profiles of carbon monoxide (CO), carbon

3 3

trioxide (CO ), and ozone (O ) were also monitored throughout the irradiation. The following

formation pathways (reactions (16-19)) are +considered for the three aforementioned species:

1

(X Σ

+

1

–

1

3

(

16)

17)

CO

2

g

) → CO (X Σ ) + O ( D / P)

1

(X Σ

1

CO

g

) + O ( D) → CO

3 1

(X A )

1

3

1

3

1

(X Σ

+

(

18)

19)

O ( P) + O ( D / P) → O

2

g

)

1

(X Σ

+

1

3

1

O

2

g

) + O ( D / P) → O (X A

3

1

)

Note that some of these reactions require intersystem crossing (ISC). Carbon dioxide is

destroyed by reaction (16), producing a carbon monoxide molecule and an oxygen atom in the

1

3

excited singlet state ( D) and/or triplet ground state ( P). This pathway is endoergic by 532 kJ

-1

mol (5.51 eV) for the triplet channel and by 732 kJ mol (7.59 eV) for the singlet channel. The

energies required to complete these processes are provided by the energy deposited from

1

4

energetic electrons passing through the ice mixture. Based on the electron fluence of 5.5 × 10

-

2

electrons cm , each electron initiates the destruction of 17 ± 6 carbon dioxide molecules,

-1

translating to 90 ± 30 eV electron or 130 ± 40 eV electron for the formation of ground and

excited state oxygen atoms, respectively. These suprathermal oxygen atoms react with

unprocessed carbon dioxide to form carbon trioxide. Using a one step A′ → B' fitting (equation

(

20-21)) the temporal profiles of carbon monoxide and carbon trioxide were modeled with the

best fits displayed in Figure 4.

ꢄCOꢊ = ꢕꢇ1 − ꢑ^ꢏ^ꢎ^ꢖ^ꢋꢈ

(

20)

21)

ꢋ

ꢄCO ꢊ = ꢗꢇ1 − ꢑ^ꢏ^ꢎ^ꢘ^ꢋꢈ

ꢆ ꢋ

1

6

-2

The result of these fittings yielded ꢕꢙ = (3.3 ± 0.2) × 10 molecules cm and ꢗ = (7.03 ± 0.06)

1

5

-2

-3 -1

×

10 molecules cm along with rate constants of ꢓ = (2.73 ± .09) × 10 s and ꢓ = (1.31 ±

ꢆ ꢚ

-

4

-1

0

.06) × 10 s . Excess oxygen atoms produced by reaction (16) can react by the pathway

described in reaction (18) to form molecular oxygen (O ) that can subsequently react with

2

another oxygen atom to form ozone (O ) as in reaction (19). Both reactions can occur with either

3

1

O ( D) or O ( P) since both lack of entrance barriers and are exogenic by 498.5 kJ mol (5.167

3

-1

-

1

eV) and 106.5 kJ mol (1.104 eV), respectively. Considering a consecutive A'' → B'' → C''

reaction scheme, the temporal evolution was modeled by the following consecutive pseudo-first

order kinetics:

^ꢎꢜ

^ꢎꢝ

(

22)

ꢄO ꢊ = ꢛ ꢍ1 −

ꢑ^ꢏ^ꢎ^ꢝ^ꢋ

−

ꢑ^ꢏ^ꢎ^ꢜ^ꢋ

ꢒ

ꢆ ꢋ

ꢎ_ꢜꢏꢎ_ꢝ

where ꢓ is the reaction rate of reaction (18) and ꢓ is the reaction rate of reaction (19). Figure 4

ꢅ

ꢞ

14

-2

shows the best fit of the ozone profile where ꢛ = (9.2 ± 2.8) × 10 molecules cm , ꢓ = (2.4 ±

ꢅ

-

4

-1

-4 -1

0

.9) × 10 s and ꢓ = (1.5 ± 0.6) × 10 s .

ꢞ

1

2

5

. Conclusion

The present laboratory study demonstrates the formation of pyridine carboxylic acids,

including the biologically relevant nicotinic acid (Vitamin B3; m-C

dicarboxylic acids (2,3-, 2,5-, 3,4-, and 3,5-), in icy mixtures of pyridine and carbon dioxide

1:8) upon exposure to ionizing radiation in the form of energetic electrons. The carboxylic acid

5 4

H NCOOH) and four

(

species unambiguously characterized within this study align closely with those identified in the

Murchison [1,2] and Tagish Lake [3] meteorites, i.e. all three monosubstituted pyridine

carboxylic acids. Our findings also found a similar suite of pyridine carboxylic acids as a more

recent investigation on a series of CM2-type carbonaceous chondrites [4] that detected all three

monosubstituted and 2,5-, 3,4-, 3,5-pyridine carboxylic acids. Additionally, our study

unambiguously identified the formation of the 2,3-dicarboxylic acid species. Collectively, the

array of pyridine carboxylic acids formed in the present experiment mimics the result of previous

studies focusing on the irradiation of pyridine-carbon dioxide and pyridine-carbon dioxide-water

ice mixtures [4,25]. However, unlike the aforementioned study, our experiments were conducted

under contamination-free ultrahigh vacuum conditions – more characteristic of interstellar

environments – with infrared data collected on line and in situ – preventing the possibility of

contamination or hydrolysis of the ice by solvents (water or water-methanol) needed for DART-

MS and LC-UV/MS analysis. The on line nature of our study also allowed for analysis of the

temporal evolution of irradiation products as identified by the development of novel bands

throughout the processing period. Underlying reaction schemes for observed products were

elucidated based on optimized fittings of the collected temporal profiles. It should be noted that

this study is a proof-of-concept study to probe possible formation routes of pyridine carboxylic

acids and further experiments with more characteristic interstellar ices, such as those including

2

water (H O), are necessary.

To model the formation of pyridine carboxylic acids mechanistically, laboratory experiments

completed on the formation of generic alkyl carboxylic acids (RCOOH) in interstellar model ices

were referenced. Kim, et al. [21] postulated that carboxylic acid formation is realized in

hydrocarbon-carbon dioxide ice mixtures upon exposure to energetic electrons through the

hyrdoxycarbonyl (HOCO) radical intermediate formed by reactions between carbon dioxide and

suprathermal hydrogen atoms lost from a hydrocarbon, R-H. The alkyl and HOCO radical may

then undergo radical-radical recombination to produce carboxylic acids. In an analogous process,

1

3

we propose that pyridine upon interaction with ionizing radiation may undergo hydrogen loss,

followed by reaction of the latter with carbon dioxide leading to the formation of HOCO radical

intermediates and subsequently pyridine carboxylic acids. Employing in situ infrared

spectroscopy, the formation of radiation-induced mono- and disubstituted carboxylic acids was

monitored along with the HOCO radical, and optimized kinetic fittings were obtained. From this,

the rate constant for the addition of a single carboxyl group was determined to be (4.4 ± 2.6) ×

-

4

-1

-4 -1

1

0 s , comparable to the 1.1× 10 s found for the formation of generic alkyl carboxylic acids

in hydrocarbon-carbon dioxide ice mixtures [21]. Addition of a second carbon dioxide molecule

-4

to a monosubtituted pyridine carboxylic acid was found to proceed at a rate of (2.7 ± 1.6) × 10

2

-1

cm s , slower than the initial addition of carbon dioxide to pyridine due to the effects of the

highly electronegative carboxyl group. Evidence for the formation of disubstituted pyridine

carboxylic acids, and subsequent mechanisms for their formation, compliment the recent

identification of these species within carbon-rich chondrites. We can also predict that if

6 5

interstellar ices contain benzene along with carbon dioxide, benzoic acid (C H COOH) should be

formed based on our mechanistical data. This assertion is supported by the discovery of benzoic

acid in the Murchison, Tagish Lake [54], and Orgueil meteorites [55].

ACKNOWLEDGEMENTS

These studies were supported by the RIK US National Science Foundation (AST-1505502) and

the W.M. Keck Foundation.

1

4

TABLES

Table 1.

Assignment of vibrational modes in pristine pyridine-carbon dioxide ice (C

5

H

5

2

N − CO ) at 10 K

-

1

Absorption (cm ) Literature Value (cm )

-1

Assignment

CO

Characterization

5

085, 4967, 4828

…

2

Combinations and

Overtones

Combination

a

b

a

b

a

b

a

b

3

710

600

089

062

044

034

3700

3600

3087

3061

3042

3030

3010

2429

2342

2282

2040

1925

1641

1598

1581

1483

1442

1384

1362

1276

1143

1071

1032

ν

1

+ν

3

CO

2

2ν

2

+ν

3

CO

2

Combination

ν

2

C

5

H

5

N

C-H Stretch

ν

13

7b

5

H

5

N

C-H Stretch

H

C-H Stretch

ν

20a

C H

5 5

N

C-H Stretch

3

009, 2995

ν

8a+ν19b

14+ν18a

C

5

H

5

N

Combination

2

1

430

328

280

044

938

639

603

C

5

H

5

Combination

ν

3

13

CO

2

Asymmetric Stretch

Combination

ν

3

CO

2

ν

6a+ν19b

9a+ν11

C

5

H

5

N

ν

C

5

H

5

N

Combination

ν

11+ν

1

C

5

H

5

N

Combination

ν

6a+ν

C

5

H

5

N

Combination

1

585, 1574

ν

8a

C H

5 5

N

Ring Stretch

1

484

441

383

358

276

147

071

032

ν

19a

C

5

H

5

N

Ring Stretch

H

N

Ring Stretch

19b

5

2ν

2

CO

2

Overtone

ν

14

5

C H

5

N

Ring Stretch

ν

1

CO

2

Symmetric Stretch

C-H Deform

ν

15

5 5

C H N

ν

18a

12

C

5

H

5

N

C-H Deform

ν

C

5

H

5

N

Trigonal Ring Breathing

Ring Breathing

Ring Twist

b

9

93

55

11

991

744

700

ν

1

C

5

H

5

N

7

ν

4

5

7

ν

11

ν

5

C H

5

N

C-H Out-of-Plane Deform

In-Plane/Out-of-Plane Bend

a

6

73, 655

38

667, 665

a

638

2 2

CO

13

CO

2

6

ν

2

Notes.

a

Bennett, et al. [34]

Wong, et al. [35]

b

1

5

Table 2.

Infrared absorption features of the irradiated pyridine-carbon dioxide ice (C

5

H

5

2

N-CO ) at 10 K and throughout heating period

Absorptions

After Irradiation

Absorptions During

Warm-Up

Literature Assignment

a

-

1

Wavenumber (cm )

10 K

2141

2044

1888

1845

−

129 K

−

208 K

293 K

−

Molecule

CO

Vibration

Characterization

Fundamental

C=O Stretch

Fermi Resonance

C=O Stretch

Ring Stretch

CH Wag, Ring Stretch

Ring Stretch

In-Plane (COH) Def

Ring Stretch

OH Scissor

b

−

2139

ν

1

b

−

2041

CO

3

1

b

−

1879

−

c

−

1811

−

1804

−

1791

1846

HOCO

ν

2

d

1790

2,5-C

2,4-C

2,5-C

2,6-C

3,4-C

5

H

3

N(COOH)

2

−

d

1

775

−

1722

−

1732

−

1741

1770

5

3

d

730

1730

1

5

3

2

d

730

e

f

1

686

1686

1688

1693

1700

704

1

ν

6

f

1

638

605

1642

1599

1648

1618

1635

1578

1640

−

g,h

1595

o-C

m-C

5

H

4

NCOOH

ν

8a

h

1

595

5

H

4

NCOOH

N(COOH)

−

d

i

610

590

610

609

2,3-C

2,4-C

2,5-C

3,4-C

3,5-C

5

H

3

2

5

H

ν

8

1

597

ν

37

j

1

555

485

1548

1486

1532

1484

1531

1487

1540

m-C

5

H

4

NCOOH

N(COOH)

ν

8

h

d

h

d

1488

480

1478

470

m-C

5

H

4

−

1

2,3-C

5

H

3

2

1

467

441

1470

1438

1469

1438

−

p-C

5

H

4

NCOOH

N(COOH)

1

2,5-C

2,6-C

5

H

3

2

e

1

470, 1468

g,h

3

1440

1439

1

o-C

m-C

5

H

4

NCOOH

ν

19b

h,j

440

443

5

H

4

NCOOH

N(COOH)

ν

9

i

j

1

3,5-C

5

H

3

2

ν

35

33

10

14

1

359

317

1346

1371

1331

1367

1323

1357

1325

5

H

ν

−

m-C

5

H

4

NCOOH

N(COOH)

f

1

317

3,4-C

5

H

3

2

d

−

032

1290

1032

1287

1283

1280

1037

2,3-C

5

H

−

k

1

−

O

3

ν

3

Asymmetric Stretch

Notes.

a

Only new absorption features and those present in ice at 298 K are reported as product peaks; carbon dioxide (CO ) and other light carbon

2

species sublimed prior to 70 K, leaving pyridine (C

heavy radiation products.

5 5 5 5

H N) and heavier radiation products; pyridine (C H N) sublimed prior to 109 K, leaving only

b

Bennett, et al. [26]

Jacox [43]

Wasylina, et al. [45]

Massaro, et al. [47]

Karabacak, et al. [48]

Lewandowski, et al. [49]

Koczoń, et al. [50]

Nataraj, et al. [46]

c

d

e

f

g

h

i

j

Kumar, et al. [44]

Bennett, et al. [51]

k

1

6

Table 3.

5 5 2

Infrared absorption features of the irradiated pyridine-carbon dioxide ice (C H N-CO ) at 10 K and throughout heating period

compared to literature valukaes for mono- and disubstituted pyridine carboxylic acids

-

Literature Values (cm )

1

Experimental

-

1 a

Absorptions (cm )

0 K

Monosubstituted

c,d

Disubstituted

1

b,c

c

e

f

e,g

h

Ortho-

…

Meta-

…

Para-

…

2,3-

…

2,4-

…

2,5-

2,6-

…

3,4-

…

3,5-

…

1

811−1790

775

1790

1770

1730

…

1

…

1

741−1722

…

1730

…

1717

1711

1712

…

1710

1704

…

1

708

…

1

693−1686

648−1635

…

1700

…

1640

…

1608

1595

…

1

618−1578

1595

1616

1597

…

1610

1580

…

1590

1610

1576

1609

1597

1

583

…

1573

…

1571

…

1

548−1531

…

1540

…

1500

…

487−1484

…

1488

…

1480

…

1478

…

470−1467

…

1470

1

468

…

441−1438

…

1455

1439

…

1

1440

1407

…

1443

1383

1412

1430

420

…

1390

1420

1400

…

1422

1387

…

1405

1

371−1346

…

1357

…

1341

…

331−1317

…

1325

…

1317

…

1310

1280

1310

…

1308

…

1298

…

290−1283

…

Notes.

a

Range of wavenumbers indicates the shift in the assigned band between the irradiated sample at 10 K and during the

warm-up.

b

Lewandowski, et al. [49]

Koczoń, et al. [50]

Kumar, et al. [52]

Wasylina, et al. [45]

Massaro, et al. [47]

c

d

e

f

g

Karabacak, et al. [48]

Nataraj, et al. [46]

h

1

7

Table 4.

Compilation of new molecules formed in pyridine-carbon dioxide ice (C

throughout heating period

5

H

5

N-CO ) at 10 K and

2

-

Absorption (cm )

1 a

-1

Literature Value (cm )

Molecule

o-C NCOOH

Assignment

b,c

5

H

4

1618−1578

1595

b,c

1439

ν

8a

1

441−1438

ν

19b

c,d

m-C

5

H

4

NCOOH

1618−1578

1595, 1583

d

1540

ν(ring)

1

548−1531

487−1484

441−1438

331−1317

ν

8

d

1488

ν(ring)

c,d

1440

1325

ν

9

d

ν

10

c

p-C

5

H

4

NCOOH

1618−1578

1616, 1597

ν(ring)

e

2

,3-C

5

H

3

N(COOH)

2

1610, 1580

e

1

487−1484

290−1283

1480

1280

ν(ring)

e

1

σ(OH)

e

2

,4-C

5

H

3

N(COOH)

2

1741−1722

618−1578

1811−1790

775

1730

1590

ν(C=O)

ν(ring)

e

1

e

,5-C

H

1790

1770

ν(C=O)

ν(ring)

e

1

e

1

741−1722

618−1578

470−1467

1730

1610

1470

e

ν(ring)

f

2

3

,6-C

5

H

3

N(COOH)

2

1693−1686

1700

ν(C=O)

ν(ring)

f

1

618−1578

470−1467

1576

1470, 1468

f

1

ρ(CH), ν(C-N)

g

,4-C

H

1693−1686

1704

1640

ν

6

g

1

648−1635

618−1578

331−1317

ν(ring)

g

1609

ν

8

g

1317

ν

14

37

35

33

h

3

,5-C

5

H

3

N(COOH)

2

1618−1578

1597

h

1

441−1438

1443

h

1

371−1346

1357

Notes.

a

Range of wavenumbers indicates the shift in the assigned band between the irradiated sample at

1

0 K and during the warm-up.

Lewandowski, et al. [49]

Koczoń, et al. [50]

Kumar, et al. [44]

Wasylina, et al. [45]

b

c

d

e

f

Massaro, et al. [47]

Karabacak, et al. [48]

Nataraj, et al. [46]

g

h

1

8

Table 5.

Rate constants and production yields of proposed reactions and new products in pyridine-carbon

5 5 2

dioxide ice (C H N-CO )

Reactions

Column Density

2

Rate Constant

-1

(s )

-

(

molecules cm )

1

5

-3

-4

CO

2

→ CO + O

+ O → CO

+ H → HOCO

O + O → O

+ O → O

→ C

N(COOH) + CO

(9.3 ± 0.8) × 10

(6.2 ± 0.2) × 10

(7.3 ± 0.9) × 10

…

(2.73 ± 0.09) × 10

(1.31 ± 0.06) × 10

…

4

3

2

3

2

-

4

2

(2.4 ± 0.9) × 10

1

4

-4

O

2

3

(9.2 ± 2.8) × 10

…

(1.5 ± 0.6) × 10_-

(4.4 ± 2.6) × 10_-

(2.7 ± 1.6) × 10

4

C

5

H

5

N + CO

2

5

H

4

N(COOH)

C

5

H

4

2

→

C

5

H

4

N(COOH)

2

1

9

FIGURE CAPTIONS

Figure 1. Nine mono- and disubstituted pyridine carboxylic acid species. Of particular interest is nicotinic acid (m-

5 4

C H NCOOH; top, center) due to its biological significance in terrestrial environments. In addition to the full suite

of monosubstituted pyridine carboxylic acids (top), 2,5- (middle, right), 3,4- (bottom, center), and 3,5- (bottom,

right) disubsituted pyridine carboxylic acids have recently been identified within a series of CM2-type carbonaceous

chondrites.

5 5 2

Figure 2. Mid-infrared spectra of the solid 10% pyridine (C H N) in carbon dioxide (CO ) ice before the irradiation

-

1

at 10 K (black). The top left spectrum displays the full spectral range of 6000 to 500 cm with the irradiated ice

shown in red. The remaining three panels depict zoomed in regions of the full spectrum. The assignment of peaks

corresponding to the two starting materials is displayed on each graph.

Figure 3. Deconvoluted infrared spectrum of ice following irradiation with energetic electrons at 10 K, 129 K, 208

-

1

K, and 293 K (from top to bottom). The two regions in which relevant bands were found, 3350-2950 cm (left) and

1

-1

950-1400 cm (right), are displayed. The red lines correspond to experimentally collected infrared spectra, the

black lines correspond to deconvoluted fits of structures within the ice, and the blue line corresponds to the sum of

the deconvoluted fittings. Absorption features assigned to radiation products are labeled by wavenumber.

Figure 4. Fit of temporal (integrated IR absorption) evolution of the two starting materials pyridine (C

carbon dioxide (CO ), and the newly formed light irradiation products carbon monoxide (CO), carbon trioxide

CO ), and ozone (O ) throughout the 1-hour irradiation period.

5 5

H N) and

2

(

3

Figure 5. Reaction scheme used to fit the temporal evolution of the identified pyridine carboxylic acids.

Figure 6. Fit of temporal (normalized integrated IR absorption) evolution of 2,5- and 3,4-pyridine carboxylic acid

throughout the 1-hour irradiation period. The profiles were fit based on a pseudo first-order, consecutive A → B → C

reaction scheme.

2

0

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On the formation of niacin (vitamin B3) and pyridine carboxylic acids in interstellar model ices

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