Cherysler Pacifica and Mercedes R350

Is the R350
based on the Chrysler Pacifica?

Last night I happened to park one next to the other and found the two vehicles too closely similar to be just paralel engineering.

No. The Pacifica is based on the Chrysler Town and Country minivan. The R350 and R500 are based on the M-class SUV platform. Any external resemblence is just a result of aerodynamics.