On a night when basketball seemed unimportant, Indiana won the most important college basketball game there is. In doing so, the Hoosiers denied North Carolina Coach Dean Smith the one honor that has escaped him during his 20 years as a head coach.

Led by the brilliant Isiah Thomas, who scored 19 of his 23 points in the second half, Indiana put together an overpowering second half to blitz the Tar Heels, 63-50, tonight, winning the national championship for the second time in six years.

But there was almost no game tonight.Because of the shooting of President Reagan this afternoon, the NCAA waited until less than an hour before tipoff to decide the contest should be played.

The tipoff was nine minutes late because of the events in Washington and the public address announcer in the Spectrum informed the crowd of 18,276 that the president had come through surgery successfully and was in good condition.

The game started and, for two hours reality stopped and a group of college kids played a game. None of the players expressed any regret over playing with Thomas summing up the general feelings of the two teams:

"We were happy the president wasn't dead. A lot of people get shot. We were just happy that he could still think with his brain, that's the most important part. We were just trying to win a ballgame tonight."

Thomas may have been trying too hard during the first half. He shot one of seven from the floor and was outplayed by Carolina point guard Jimmy Black. As a team the Hoosiers, who won their last 10 games to finish 26-7, could not get untracked offensively, shooting just 38 percent.

But offense does not make or break Indiana teams. It is defense that has made Coach Bob Knight a national champion twice. Tonight was no different. For the first 20 minutes the Indiana defense kept it in the game. The second 20 minutes the Indiana defense took North Carolina out of it.

"They played more aggressively on defense tonight than the two times we played them in the regular season the last two years," said Smith, who refused to show any emotion at the end. "They played about as good a second half as anyone has played against us all season."

It was during the first half, though, that Carolina had it chance to win this game. Al Wood, who had destroyed Virginia Saturday with 39 points, started well, hitting three of his first five shots, making it clear to Knight that Landon Turner could not stay with him.

Into the game came Jim Thomas. At 6 feet 3, he gave away three inches to Wood. But he made up for it with quickness. Wood finished the night with 18 points, but many of them came in the final three minutes when Indiana was in control as Thomas dogged him over the court.

"I knew I had the quickness and I knew I would get help from my teammates," said Thomas. "I tried to deny him the ball, keep him from getting that first step to the hoop."

With Wood quiet, the Tar Heels struggled offensively. They also had problems because Smith was forced to rest Wood, Sam Perkins and James Worthy. "I was getting a lot of tired signs out there," Smith said. "I think we're in shape. We might have been too excited."

The tired signs and Thomas kept the Tar Heels from building on a 16-8 lead midway through the half. Indiana crept back, Randy Wittman doing most of the offensive work from outside. On the final play of the half, Wittman hit his fourth outside bomb and it gave Indiana a 27-26 halftime lead.

"That shot was important because it meant we were ahead even though we knew we hadn't played a good half," Isah Thomas said. "It may have been the turning point of the game."

Not so, said the two coaches. "I thought the key for us were the two quick steals Isah got at the start of the second half," Knight said. "That got us going."

Smith agreed. "If there was a turning point, those two steals, were it," he said.

For Thomas the turning point may have come during intermission. He sat in front of his locker listening as Knight told his team, "Patience, be patient. We can play much better. Just think patience and remember there are only 20 minutes left in the season."

Quickly, Thomas made certain this season would end for Indiana the same way it did the last time the final four played here. in 1976. On the first possession of the second half he stole the ball from Black and went in for a layup and a 29-26 lead. Perkins and Turner traded baskets then Thomas did his magic act again. He dashed into the middle, flicked the ball from Perkins and was gone again for another layup and a 33-28 lead.

Carolina, which almost never gets rattled, was rattled. Worthy, who made just three of 12 shots before fouling out, threw another bad pass and Wittman, who finished with 16 points, hit another jumper. Black finally hit but Thomas made a jumper and went backdoor for a layup. In just 4:28 he had eight points, two assists and two steals. Indiana led, 39-30, and Carolina never get closer than seven.

"Isiah getting going that way is the way it's been all season," said center Ray Tolbert, who had 11 rebounds. "He's our catalyst. He makes us roll."

The Hoosiers rolled this entire tournament, winning their five games by 113 points, a record. They shot 63 percent tonight, compared to 43 percent for the Tar Heels (36 percent the second half). They made an excellent team, one that finished its season 29-8, look bad.

"They took us out of our offense competely," said Wood, who wept during the award ceremonies. "They're a lot like us, only more patient. That may be the big difference, patience."

Carolina's impatience was created by Indiana's in-your-face, help-at-every-turn defense. The Hoosiers overplayed everywhere, forced 19 turnovers and only lapsed once, letting Carolina creep within 55-47 with 2:22 left on a Mike Pepper jumper.

But just when it appeared, as Smith put it, "that we might have a great TV finish," Indiana nailed the lid shut. With Carolina trying to close the margin to six, Tolbert stole the ball from Perkins and fed Risley, who was fouled and made both shots.

That sealed it. Indiana, which had been 7-5 at one time in December, had won a title even Knight never imagined it could until the last month of the season.

For Knight and his players, the events in Washington did little to dampen that joy. Knight said nothing to his players about the shootings, knowing they were aware of them. Instead he brought Quinn Buckner, captain of the 1976 team and John Havlicek, his teammate on the 1960 Ohio State championship team in to talk to the players about what winning a national title meant.

For Knight and the Hoosiers it meant joy and ecstasy. For Smith and his players it was another bitter pill, an almost horrifying letdown after being so close.

Denied the national title in the ultimate game for the third time, Smith remained stoic. He applauded his players as they received their awards and showed them no emotion during their brief time alone in the locker room.

Only once did Smith betray his feelings. When someone asked if he felt frustrated at falling short again, he showed his frustration by feigning happiness.

"I guess we'll be like Penn State football," he said. "They finish second sometimes, too. I'm thrilled to be second. Ask Digger (Phelps) how he would feel about being second. He's only been to the final four once. Ask my great friend Looie (Carnesseca). He's never been here.

"I'm thrilled to be No. 2."

Less true words were never spoken.