Context : In the high-mass star-forming region G35.20-0.74N , small scale ( \sim 800 AU ) chemical segregation has been observed in which complex organic molecules containing the CN group are located in a small location ( toward continuum peak B3 ) within an apparently coherently rotating structure .

Aims : We aim to determine the physical origin of the large abundance difference ( \sim 4 orders of magnitude ) in complex cyanides within G35.20-0.74 B , and we explore variations in age , gas/dust temperature , and gas density .

Methods : We performed gas-grain astrochemical modeling experiments with exponentially increasing ( coupled ) gas and dust temperature rising from 10 to 500 K at constant H _ { 2 } densities of 10 ^ { 7 } cm ^ { -3 } , 10 ^ { 8 } cm ^ { -3 } , and 10 ^ { 9 } cm ^ { -3 } .
We tested the effect of varying the initial ice composition , cosmic-ray ionization rate ( 1.3 \times 10 ^ { -17 } s ^ { -1 } , 1 \times 10 ^ { -16 } s ^ { -1 } , and 6 \times 10 ^ { -16 } s ^ { -1 } ) , warm-up time ( over 50 , 200 , and 1000 kyr ) , and initial ( 10 , 15 , and 25 K ) and final temperatures ( 300 and 500 K ) .

Results : Varying the initial ice compositions within the observed and expected ranges does not noticeably affect the modeled abundances indicating that the chemical make-up of hot cores is determined in the warm-up stage .
Complex cyanides vinyl and ethyl cyanide ( CH _ { 2 } CHCN and C _ { 2 } H _ { 5 } CN , respectively ) can not be produced in abundances ( versus H _ { 2 } ) greater than 5 \times 10 ^ { -10 } for CH _ { 2 } CHCN and 2 \times 10 ^ { -10 } for C _ { 2 } H _ { 5 } CN with a fast warm-up time ( 52 kyr ) , while the lower limit for the observed abundance of C _ { 2 } H _ { 5 } CN toward source B3 is 3.4 \times 10 ^ { -10 } .
Complex cyanide abundances are reduced at higher initial temperatures and increased at higher cosmic-ray ionization rates .
Reaction-diffusion competition is necessary to reproduce observed abundances of oxygen-bearing species in our model .

Conclusions : Within the context of this model , reproducing the observed abundances toward G35.20-0.74 Core B3 requires a fast warm-up at a high cosmic-ray ionization rate ( \sim 1 \times 10 ^ { -16 } s ^ { -1 } ) at a high gas density ( > 10 ^ { 9 } cm ^ { -3 } ) .
The abundances observed at the other positions in G35.20-0.74N also require a fast warm-up but allow lower gas densities ( \sim 10 ^ { 8 } cm ^ { -3 } ) and cosmic-ray ionization rates ( \sim 1 \times 10 ^ { -17 } s ^ { -1 } ) .
In general , we find that the abundance of ethyl cyanide in particular is maximized in models with a low initial temperature , a high cosmic-ray ionization rate , a long warm-up time ( > 200 kyr ) , and a lower gas density ( tested down to 10 ^ { 7 } cm ^ { -3 } ) .
G35.20-0.74 source B3 only needs to be \sim 2000 years older than B1/B2 for the observed chemical difference to be present , which maintains the possibility that G35.20-0.74 B contains a Keplerian disk .