Rachel Bilson soon single?

'The O.C.' stars Adam Brody and Rachel Bilson are reportedly on the verge of splitting up.Sources close to the couple - who got together while making the hit TV show - claim Rachel's blossoming friendship with 'Scrubs' actor Zach Braff, who she stars with in new movie 'Last Kiss', is the reason for the problems in the pair's relationship.One source told America's Star magazine: "They looked like they were on their way to getting engaged just a little while back but they are going through a really difficult period right now."Zach calls Rachel a lot and they talk on the phone like girlfriends. What really eats away at Adam is when Rachel starts talking about how mature Zach is. No guy appreciates his girlfriend comparing him to another guy."Rachel and Zach had to film racy sex scenes together for their new movie.The brunette actress recently admitted she was really nervous before shooting the scenes - because she and Zach barely knew each other.She said: "I was really nervous because this was my first big sex scene. I play a temptress who pursues Zach's character. It was a little awkward, it was like, 'Hello, nice to meet you, let's get into bed.'"